Microalbuminuria is a reliable predictor of the eventual development of overt diabetic nephropathy and blood pressure is known to accelerate the course of this nephropathy. In the present studies, the effect of a 6-week treatment by placebo (n = 7), nifedipine (n = 7) and captopril (n = 8) on renal function and urinary excretion of albumin (UAE) was investigated in normotensive, insulin-dependent, diabetic patients with incipient nephropathy (UAE greater than 15 micrograms/min). No change in arterial pressure, renal function or UAE was observed in the placebo group. In response to captopril and nifedipine, mean arterial pressure decreased slightly and similarly in both groups. Glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) increased to a similar extent in the nifedipine group, thus resulting in no change in filtration fraction (FF). In response to captopril, GFR was unchanged whilst ERPF increased; as a consequence FF decreased. Opposite changes in UAE were observed in response to the two treatments; UAE decreased by 40% in the captopril group and by 40% in nifedipine-treated patients. These results indicate that intrarenal changes may be crucial with respect to the effect of therapy on UAE. It is suggested that only agents which reduce FF and probably intraglomerular capillary pressure, such as converting enzyme inhibitors, alter UAE and may possibly interfere with the course of incipient diabetic nephropathy in normotensive patients.