Relevance. Every year, dog breeders are faced with the problem of acute poisoning of dogs. Various toxic compounds can be the cause of dog poisoning. The huge variety of potentially toxic substances makes it difficult to identify the source of poisoning. Service dog breeding also faces this problem, as a result of which it is necessary to have information about the possibility of poisoning service dogs, as well as methods for detecting toxic substances and preventing further poisoning. In service dog breeding, they mainly use feed made by cooking gruel soup in boilers in the feed kitchens of nurseries. Due to the fact that there were cases of poisoning of service dogs in the Vyborg district of the Leningrad region, we conducted research to identify toxic compounds in beef from which feed was made in the feed kitchens of nurseries.Methods. The studies were carried out at the Institute of Toxicology of the Federal Medical and Biological Agency using an Acguity UPLC I-class ultra-performance liquid chromatograph with a spectrophotometric detecto and an Acguity UPLC H-class ultraperformance liquid chromatograph with a Xevo TQD tandem mass spectrometer. Sample components were identified by electronic spectra and mass numbers.Results. According to the results of the study, the presence of isoniazid metabolic products in the form of isonicatinic acid, as well as conjugates of isoniazid with sulfuric, acetic and glucuronic acids, was found in beef meat.The obtained results of the study of the chromatographic profiles of aqueous extracts and the chemical identification of their components in the products that are used for the preparation of food for service dogs make it possible to accurately determine and identify toxic substances with a wide variety of them. Thus, in order to prevent the loss of service dogs, it is necessary to control the newly received food and feed in order to prevent poisoning.