Have you ever watched your cat sniff something, pause and then sit with their mouth slightly open? The expression can look like a grimace, a sneer or complete shock at whatever they just smelled.
Your cat is not disgusted. They are displaying the flehmen response, a normal behavior that helps cats gather more information about an interesting scent.
By holding their mouth open, cats can direct scent molecules toward a special sensory structure called the vomeronasal organ, better known as Jacobson’s organ.
The flehmen response is the unusual open-mouth expression cats sometimes make after smelling something interesting.
A cat displaying the flehmen response may:
It may look as if your cat is smelling with their mouth. In a way, that is exactly what is happening.
The flehmen response helps move scent molecules toward Jacobson’s organ so the cat can examine chemical information that ordinary sniffing may not fully reveal.
Jacobson’s organ is another name for the vomeronasal organ, or VNO. It is a specialized sensory structure that helps many animals detect pheromones and other chemical signals.
In cats, Jacobson’s organ works alongside the regular sense of smell. The nose detects everyday airborne odors, while the vomeronasal system helps analyze certain chemical messages more closely.
These chemical signals can give cats information about:
Scientific research describes the vomeronasal organ as part of an accessory olfactory system involved in chemical communication and social behavior.
A cat’s paired vomeronasal organs are located above the roof of the mouth, near the nasal cavity.
Small ducts behind the upper front teeth help carry scent-containing material toward the organ. When a cat performs the flehmen response, the position of the mouth and upper lip helps move those chemical compounds into the vomeronasal system.
This is why the cat pauses with an open mouth instead of continuing to sniff normally.
Cats open their mouths to investigate an odor more thoroughly.
The behavior is commonly triggered by scents containing information from another animal. Your cat may display the flehmen response after smelling:
Both male and female cats can display the response. It is not limited to unneutered males or mating behavior.
The classic flehmen face usually begins with focused sniffing. The cat may then stop, raise their head and hold their mouth open.
Some cats expose their front teeth. Others simply part their lips and appear to freeze for a few seconds.
The expression may look like your cat is:
Despite the dramatic expression, the cat is usually calm and concentrating on the scent.
Pheromones and scents left by other animals are common triggers. Cats use chemical communication more than people realize.
Cats leave scent messages by:
Urine marking is one way cats leave chemical messages for other animals. If you smell urine but cannot locate the source, use these tips to find hidden cat pee in your home. Once you locate the affected area, follow our guide to removing cat pee smell from carpet, furniture and flooring
Your cat may investigate these scent messages to learn who was present and whether the smell is familiar.
A cat may also perform the flehmen response after smelling something new or unusually strong, even when another animal is not involved.
Cats sniff one another to gather information and recognize familiar members of their social group.
The face, head, sides and rear of another cat can all carry meaningful scent information. Cats may also rub against each other to exchange familiar group scents.
If your cat sniffs another cat and then makes an open-mouth expression, they may be processing the scent through Jacobson’s organ.
This does not necessarily mean the cats are upset or preparing to fight. Look at the rest of their body language. Relaxed ears, a neutral tail and loose posture generally indicate curiosity rather than conflict.
Your face carries a combination of familiar personal scents, products and smells collected throughout the day.
A cat may sniff your nose or face because:
Cats also have scent glands around their cheeks and forehead. When your cat rubs their face against you, they may be depositing familiar scent as part of social bonding.
Yes. The flehmen response is a normal feline behavior.
Some cats display it frequently, while others do it only occasionally. You may notice it more in a multi-cat household, after returning from somewhere with other animals or when introducing new objects into the home.
The behavior itself does not require treatment.
No. The flehmen response and panting look different and have different purposes.
During the flehmen response, a cat usually:
A panting cat may:
Persistent open-mouth breathing in a cat is not a normal flehmen response. Contact a veterinarian promptly if your cat is panting without an obvious, brief explanation or appears weak, distressed or unable to breathe comfortably.
Cats have a highly developed sense of smell that helps them navigate their environment, recognize other animals, locate food and detect changes around them.
Their scent abilities include two connected systems:
This combination allows cats to gather information people cannot detect simply by looking at an object or another animal.
Rather than relying on a specific claim that a cat’s sense of smell is a certain number of times stronger than ours, it is more accurate to say that scent plays a much larger role in how cats understand their surroundings.
Scent helps cats create a map of what is safe, familiar and potentially threatening.
This is one reason cats may react strongly when furniture is replaced, a new pet arrives or familiar objects are cleaned with a strongly scented product. The environment may look the same to us while smelling completely different to them.
Cats may restore familiar scents by:
Scratching is another important form of feline communication. Learn more about why cats love to scratch and what the behavior communicates.
These behaviors help make their environment feel recognizable and secure.
The flehmen response is the open-mouth expression a cat makes while analyzing an interesting scent. It helps direct scent molecules toward Jacobson’s organ.
The expression may look like disgust, but your cat is usually concentrating on the scent. The curled lip and open mouth are part of the flehmen response.
Yes. Both male and female cats use Jacobson’s organ to process chemical signals.
Kittens can display the flehmen response as their sensory and social behaviors develop. You may notice it when they encounter unfamiliar animals or scents.
Jacobson’s organ is connected to scent detection but is located near the nasal cavity above the roof of the mouth. Small ducts help carry chemical compounds toward it.
Your cat may be using the flehmen response to analyze chemical signals left by the other cat. Those signals can carry information about identity, territory and reproductive status.
A brief, still open-mouth expression following focused sniffing is usually normal. Continued open-mouth breathing, rapid breathing or signs of distress are different and warrant veterinary attention.
When a cat sniffs something and holds their mouth open, they are probably displaying the flehmen response.
That funny face helps move scent molecules toward Jacobson’s organ, allowing the cat to examine chemical information that humans cannot detect. It is a normal and fascinating example of how heavily cats rely on scent to understand their world.
Explore more of our cat care and behavior guides to better understand your cat’s health, habits and wonderfully strange behavior.
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