Rat recipients of renal allografts and unilaterally nephrectomized control rats were studied to evaluate the response in blood pressure to prednisolone in diverse doses, and to determine the dosage required to achieve adequate immunosuppression without undue complication of hypertension. While a continuous infusion of 2 mg/kg/day or more of prednisolone proved effective in prolonging allograft survival time, this dosage increased the blood pressure of recipients as well as unilaterally nephrectomized control rats. In contrast to control rats, the recipients remained hypertensive after the cessation of prednisolone administration. This suggests that the high blood pressure observed during prednisolone administration was due to its hypertensive action. On the other hand, the high blood pressure remaining after cessation of the prednisolone administration is likely to be caused by an incomplete prevention of rejection. If recipients had received a transfusion of donor-strain blood prior to transplantation in combination with the infusion of 2 mg/kg/day or more of prednisolone, they became normotensive when the prednisolone infusion was ceased. By reducing the prednisolone dosage to 1 mg/kg/day in combination with donor-strain blood pretreatment, hypertension could also be eliminated during the first two weeks. In conclusion, effective immuno suppression can be achieved with prednisolone in rats, without inducing hypertension, provided prednisolone is administered at a low dosage in combination with adjuvant immunosuppression--i.e., donor-strain blood pretreatment.