Mercury is an element that exhibits high toxicity even in small concentrations, and therefore its content in the environment, feed and food products is strictly specified by sanitary and hygienic standards. In long-lived predatory fish, the accumulated mercury content can exceed its content in the habitat by hundreds of thousands and even millions of times. Therefore, from the point of view of the impact on animal health, it poses the greatest danger. Pets, which are traditionally fed fresh or thermally processed sea fish, are at especially high risk. According to some studies in animals, mercury poisoning is manifested by damage to the liver, kidneys and pathology of the central and peripheral nervous system. Morphological and biochemical blood tests for this poisoning remain uninformative, so toxicological studies are required. In a veterinary clinic, in order to establish a diagnosis and treat an animal, a case of chronic mercury poisoning of a cat when feeding sea fish was laboratory confirmed. The article presents the results of a clinical examination of the animal, the results of laboratory tests, the method of treatment and recommendations for the prevention of mercury poisoning.