Cats normally pant to lower their body temperature when they are overheated. Cats can also pant as a response to fear, anxiety, stress or medical conditions such as a respiratory or cardiovascular disorder.
Panting can be a normal reaction to cool the body temperature after the cat has exercised or overheated. As the cat holds his mouth open and breathes, excess water is evaporated from the tongue and mouth, and the warm air in the lungs is exchanged for cooler air outside the body. If rapid panting is accompanied by discomfort or noticeable labored breathing, a heat stroke could be the cause, and immediate vet assistance may be needed. Cats may also pant when they experience an environmental change that triggers stress or anxiety, such as a car ride or introduction to a new home.