This paper presents the results of research of hematologic parameters of male Wistar rats 1, 2, 3, 4 and 6 months age, which received a balanced semisynthetic diet. Studies were carried out at the Hematology analyzer Coulter AC TTM 5 diff OV (Beckman Coulter, USA) with the program, specially developed for the study of rats' blood. According to the results of research, was found a statistically significant increased of the number of red blood cells; the concentration of hemoglobin and hematocrit in animals 2-6 months compared with rats, 1 month age. With age, there is a decrease of the mean corpuscular volume and the mean corpuscular hemoglobin. The number of white blood cells in rats of 2-4 months age are significantly higher than in rats of 1 and 6 months age. The number of neutrophils and eosinophils in rats of to the 2 month are of is lover than once in rats of 1 month age, and increases values in animals of 6 months age. The number of lymphocytes has the highest value in the rat of 2-3 months age and the minimum value is that in animals of 6 months age. With increasing of the age of the animals the reduction of contents of monocytes was noted. The content of platelets and the platelet crit in the blood of rats 6 months age is statistically greater than those in 1-month age animals. The average volume of platelet is the stable index, with age does not change.