For the acute rejection after kidney homotransplantation in dogs when 20 mg/kg/day of MPSS (methylprednisolone) had been administered for 3 days, serum creatinine recovered from 3.4 mg/dl to 2.4 mg/dl, and the rejection was successfully inhibited in 13 cases (93%) out of 14. By histological findings 4 out of 6 cases showed a marked improvement of cell infiltration, 2 cases a slight improvement, and there was not a single case showing any increase of cell infiltration. As the complication of MPSS administration, there could be observed pneumonia and hemorrhage in the upper alimentary canal. Pneumonia occurred in 4 cases (29%) out of 14, of them 3 cases (21%) died from pneumonia. Hemorrhage in the upper alimentary canal was observed in 2 cases (14%) of the 14 studied.