Cats most often hold their mouth open after smelling something to pull scent molecules into a specialized organ on the roof of their mouth. This behavior, called the flehmen response, looks strange but is completely normal. That said, a cat holding its mouth open for prolonged periods, especially while breathing, can signal a medical problem that needs attention.
The Flehmen Response
When your cat sniffs something and then freezes with its mouth hanging open, lips pulled back, and a glazed look in its eyes, it’s using a sensory system most people don’t know exists. Cats have a structure called the vomeronasal organ (or Jacobson’s organ) located in the roof of the mouth, with openings just behind the upper front teeth. By holding the mouth open and curling the tongue, a cat directs air upward into this organ, which analyzes chemical signals that the regular nose can’t fully decode. The sensation has been described as tasting and smelling at the same time.
The vomeronasal organ is tuned to pick up pheromones and hormones from other animals. These chemical cues carry detailed information: whether another cat is nearby, whether it’s a potential mate, whether an animal in the area is stressed or calm, and even territorial boundaries. Cats can also use it to detect prey. The signals travel from the organ to the scent-processing area of the brain, giving the cat a richer picture of its environment than smell alone provides.
Common Triggers Around the House
You’ll most often catch the flehmen response when your cat investigates another animal’s scent, like a spot where a neighborhood cat sprayed or a dog’s bedding. But plenty of household smells trigger it too. Dirty laundry, shoes, a guest’s bag, or even your own skin after exercise can set it off. Your cat isn’t disgusted. It’s gathering information. The open-mouth pause typically lasts a few seconds, and the cat returns to normal immediately afterward.
Panting From Stress or Heat
Unlike dogs, cats rarely pant. Dogs pant routinely to cool down because they can’t sweat through fur-covered skin. Cats also have limited sweat glands (only between their paw pads), but panting is not their go-to cooling method. When a cat pants, it usually means the cat is stressed or overheating.
Short bursts of open-mouth breathing can happen during a stressful event like a car ride or a vet visit and will typically stop once the cat calms down. Heatstroke is a more serious concern: it occurs when a cat’s body temperature rises above 105.8°F. If your cat is panting in a hot environment and seems lethargic or disoriented, it needs to be cooled down and seen by a vet quickly.
Respiratory and Heart Problems
Sustained open-mouth breathing in a cat is a red flag. A healthy resting cat should not exceed 35 breaths per minute. If your cat is breathing with its mouth open, flaring its nostrils, or changing its posture to get more air (stretching its neck forward, for example), something is interfering with normal breathing.
One cause is hypertrophic cardiomyopathy, the most common heart disease in cats. When this condition progresses to congestive heart failure, fluid accumulates in or around the lungs. Affected cats show labored or rapid breathing, open-mouthed breathing, and lethargy. Flat-faced breeds like Persians and Exotics face an additional risk: brachycephalic obstructive airway syndrome. Their shortened muzzles can come with narrowed nostrils, undersized nasal passages, and an elongated soft palate, all of which restrict airflow. This can cause chronic noisy breathing, exercise intolerance, and in severe cases, fainting.
Dental Pain
A cat that holds its mouth open or partially open while eating, drools, or turns its head to one side while chewing may be dealing with dental disease rather than analyzing a scent. Tooth resorption is the most common cause of tooth loss in cats, affecting between 30% and 70% of the population. It’s painful, and cats with this condition often become reluctant to eat or visibly irritable.
A more severe condition called stomatitis involves widespread inflammation and sores throughout the mouth’s soft tissue. Cats with stomatitis may stop eating entirely, drool heavily, or develop noticeably bad breath. Treatment often involves extracting most or all of the teeth, which sounds drastic but produces good results. Studies show that about 70% to 80% of cats experience substantial improvement or complete resolution after extraction, with roughly half achieving a clinical cure within about five weeks. The 20% to 30% of cats that don’t improve significantly with extractions alone typically require ongoing medical management.
How to Tell the Difference
The context matters more than the behavior itself. A cat that freezes with its mouth open for a few seconds after sniffing a shoe is performing the flehmen response. A cat that holds its mouth open repeatedly or continuously, especially while resting, is showing a different kind of signal. Key distinctions to watch for:
- Duration: The flehmen response lasts seconds. Medical open-mouth breathing persists.
- Breathing effort: A cat doing the flehmen response is still and focused. A cat in respiratory distress shows visible effort with each breath, nostrils flaring, sides heaving, or posture changes.
- Eating changes: If the open mouth comes with drooling, head tilting while eating, or food avoidance, dental pain is likely.
- Other symptoms: Lethargy, weight loss, coughing, or blue-tinged gums alongside open-mouth breathing point to a heart or lung problem.
The flehmen response is one of the more entertaining quirks of cat behavior, a brief, blank-faced pause that means your cat is simply reading invisible chemical messages in the air. Persistent or labored open-mouth breathing is a separate issue entirely, and in cats, it almost always warrants a closer look.