On the model of premature aging induced by 30-day dark deprivation (light-dark cycle 24/0 h, artificial illumination 300 Lux), the effect of exogenous melatonin (0.3 mg/kg, 14 days after the cancellation of dark deprivation, intragastric dailyin 2 % starch mucus) on the dynamics of body weight and the level of Klotho protein in the blood was studied in outbred white male rats of 4 months of age. It has been shown that prolonged dark deprivation is accompanied by a statistically significantdecrease in the content of the Klotho protein in the blood serum, which is a marker of premature aging. Metabolic disorders associated with a statistically significant decrease in the level of melatonin and Klotho protein in the blood were expressedin fluctuations in the dynamics and rate of body weight gain in animals. At the same time, exogenous melatonin had a protective effect on the condition of animals –it contributed to an increase in the content of Klotho protein in the blood serum and restoredthe level of melatonin to the values of the negative control indicator.