Cats open their mouth after smelling something to pull scent particles into a specialized organ on the roof of their mouth called the vomeronasal organ (or Jacobson’s organ). This behavior has a name: the flehmen response. It looks like your cat is grimacing, sneering, or frozen mid-gasp, but it’s actually a deliberate second layer of smell analysis that goes far beyond what their nose alone can do.
How the Flehmen Response Works
Cats have two separate systems for detecting scent. The first is ordinary smell, handled by receptors deep inside the nasal cavity. The second is the vomeronasal system, which uses a small, fluid-filled organ located just behind the upper front teeth, connected to the roof of the mouth by a tiny duct.
When a cat encounters a particularly interesting scent, it will typically press its nose or mouth directly against the source first. Then it lifts its head, curls back its upper lip, and holds its mouth slightly open. This posture channels air (and the chemical particles dissolved in it) up through the duct and into the vomeronasal organ. The organ’s interior is lined with specialized receptor cells that can detect chemicals ordinary smell receptors miss, particularly pheromones and hormones left behind by other animals.
The pose is held for just a few seconds before the cat returns to normal. It can look like panting, disgust, or a blank stare of confusion, which is why it’s sometimes called “stinky face.” But the cat isn’t reacting to a bad smell. It’s concentrating.
What the Vomeronasal Organ Detects
The vomeronasal organ is tuned to chemical signals that carry social information. Pheromones are the main target. These are chemicals other cats secrete to mark territory, signal that they’re ready to mate, or help a mother keep track of her kittens. When your cat flehms over a spot on the floor, a shoe you brought in from outside, or another animal’s bedding, it’s reading a chemical message that its regular sense of smell can’t fully decode.
This is why the flehmen response almost always follows direct nose-to-surface contact with the source material. The cat needs to pick up fluid-borne chemical traces, not just airborne ones. Urine marks from other cats are one of the most reliable triggers, but cats will also flehm in response to unfamiliar scents on clothing, new objects in the home, or spots where other animals have been.
A Separate Pathway to the Brain
What makes this system genuinely distinct from normal smell is that the signals take a completely different route through the brain. Regular odors travel from the nasal cavity to the main olfactory bulb and then to areas involved in conscious scent recognition, like the piriform cortex. Signals from the vomeronasal organ bypass that path entirely. They travel through the vomeronasal nerves to the accessory olfactory bulb, and from there to a cluster of brain structures collectively called the vomeronasal amygdala, which includes regions tied to hormonal responses, mating behavior, and territorial processing.
In practical terms, this means the flehmen response feeds information into the parts of the brain that govern instinct and social behavior rather than the parts that simply identify “what is that smell.” Your cat isn’t just smelling more intensely. It’s running the scent through a fundamentally different analysis system.
When Kittens Start Doing It
Kittens begin showing a rudimentary version of the flehmen response around five weeks of age. By seven weeks, the behavior looks essentially the same as it does in adult cats. This early development makes sense given how central pheromone communication is to feline social life. Even very young kittens need to process the calming pheromones produced by nursing mothers, which help them feel secure and oriented in their environment. Commercial pheromone diffusers for cats are designed to mimic exactly these signals.
Flehmen vs. Breathing Problems
The open-mouth pose of the flehmen response can look similar to panting or open-mouth breathing, which sometimes causes concern. The distinction is straightforward. A flehmen response is brief, lasting only a few seconds, and is always triggered by sniffing something specific. The cat’s body is relaxed, its head is typically raised, and it returns to normal quickly.
Open-mouth breathing from respiratory distress looks different. It tends to be continuous or recurring, often accompanied by visible effort in the chest or belly, noisy breathing, lethargy, or a change in gum color. A cat that holds its mouth open without having just investigated a scent, or one that does it repeatedly over minutes rather than seconds, is showing a symptom rather than a behavior. Cats are obligate nose-breathers under normal conditions, so sustained mouth breathing is always worth taking seriously.
Why Your Cat Does It at Home
Indoor cats flehm more often than many owners realize. Common household triggers include shoes and bags that carry outdoor scents, spots where another pet has rested, laundry with strong body odor, certain plants, and grocery bags. Intact (unneutered) cats tend to flehm more frequently, especially around urine marks, because they’re processing reproductive pheromones. But neutered cats do it too. Any sufficiently novel or chemically rich scent can prompt the response.
If your cat pauses over a particular spot on the carpet, curls its lip, and stares into the middle distance with its mouth hanging open, it’s not broken. It’s gathering intelligence. The flehmen response is one of the more sophisticated sensory tools in a cat’s arsenal, and the slightly ridiculous face it produces is just a side effect of the plumbing.