Low basal and stimulated plasma renin activity (PRA) levels have been reported in patients with diabetic nephropathy (DN). We have measured PRA before and after stimulation with captopril in 28 patients with DN and in 25 control patients. Renal function impairment was similar in both groups. Most patients were treated with furosemide. In 19 patients with DN the PRA-response to dihydralazine was also studied. PRA before and after captopril were higher in the DN than in the control group (p less than 0.001). PRA increased from 4.6 +/- 3.6 to 6.3 +/- 5.3 in the DN, and from 1.8 +/- 2.7 to 2.7 +/- 3.4 in the control patients. The increases in PRA, caused by decreased angiotensin II feed-back inhibition, were comparable. PRA did not increase after dihydralazine despite a pronounced blood-pressure reduction. The difference in response to these stimuli indicate selective lesions involving both the sympathetic innervation and the renal baroreceptor function in DN. Overhydration is a plausible explanation to the low PRA earlier reported in DN. The results thus indicate that a preserved renin secretion capacity is present in DN. Differences in PRA between the both groups can only partly be explained by other factors than DN. Our findings indicate a role for the reninangiotensin-system in hypertension in DN.