After conversion from cyclosporine (Cys) only to prednisolone and azathioprine four months after cadaveric renal transplantation, effective renal plasma flow (ERPF), glomerular filtration rate (GFR) and filtration fraction (FF) all improve. However, this improvement is not uniform. GFR and FF improve in all patients after one week of combined Cys and prednisolone treatment. ERPF improves under the same circumstances only in recipients without previous rejection episodes. After discontinuation of Cys and addition of azathioprine ERPF improves further in all patients. These findings suggest the presence of a low grade rejection, together with Cys nephrotoxicity. The conversion procedure seems to be safe at least during a follow-up period of 9-14 months.