Melatonin (MT), a hormone synthesized in the pineal gland, regulates a number of physiological processes in mammals. The range of clinical applications of the synthetic analogue of the hormone in both humans and animals is increasing. This paper, based on our observations, describes the applicability of MT in tablets in various cases of abnormal behavior of cats and dogs. Cases in which the efficacy of MT was confirmed include, among others, the animal's anxiety caused by excessive noise, by being left alone at home, and by transportation. Other disturbances in which the drug can be applied include excessive excitability of cats and dogs in the postoperative period resulting in breaking the stitches or licking fresh surgical wounds, as well as various types of stereotypies, such as the animal chasing its own tail, dermatitis caused by obsessive licking, or aggression towards other animals or the owner. An additional argument in favor of applying MT in cats and dogs is its virtual non-toxicity.