Seven healthy dogs were administered with methylprednisolone orally (2 mg/kg body weight/d) at the beginning of the study (between 0-10 d), then the dosing was decreased at 10 d intervals throughout 10 to 60 d in an alternate-day therapy programme. Drug administration to the dogs was stopped on the 60 d. Total leukocyte count; polymorphonuclear leukocytes, and neutrophil ratios increased and eosinophil, mononuclear leukocytes, lymphocyte, and monocyte ratios were decrease when these values were compared to changes between days. Furthermore, in lymphocyte subsets, B lymphocytes and CD 8 T lymphocytes ratios were found to decrease. In addition, CD 3 T lymphocyte ratios decreased on the 61 d and CD 4 T lymphocyte ratios increased on the 21, 41, 51 and 61 d. IgM and IgG levels decreased between the days. Serum total ADA and ADA 1 activities decreased during the application, but ADA 2 activity did not change significantly. As a result, after immunosuppressive dose of methylprednisolone application, reduction in the ADA and ADA 1 activity can be used as determinant of the suppression of immune system components.