Cats are generally homebodies by nature so bringing them to the veterinarian can be stressful. One thing that can make the trip easier is a carrier that the top can be easily removed. This allows them to stay in the bottom half and can help cats (and small dogs) feel more secure and less fearful. It is also helpful to leave the carrier out at all times and place treats in it, so your cat becomes accustomed to it. Synthetic phermone sprays such as Feliway can also be calming.
