This work is devoted to the study of metabolic changes in dogs and cats with chronic renal failure (CRF) using diet–therapy during their treatment. One of the main indicators of the progression of chronic renal failure in small animals is creatininemia, hypertension and proteinuria. Diagnosing them in small animals with chronic renal failure is an important step to establish stages and substages of the disease according to International Renal Interest Society. In order to assign the appropriate treatment, monitoring it and predict the course of the disease, it was diagnosed the stages of CRF by determining the serum creatinine concentration. Along with this, there is an important determination of CRF substages depending on indices of proteinuria (clarifying its causes) and blood pressure that indicates the degree of target–organ damage. Hence, the main changes in the metabolism of the organism of small animals with chronic renal failure relate mainly metabolic processes of protein, and water–mineral metabolism. It has been determined that, during treatment, feeding cats and dogs on diet Royal Canin «Renal» (based on a low protein concentration, the required content of electrolytes, vitamins, and lipids) decreases the time of occurrence of sustained remission of animals in 4 times.