We studied plasma renin activity (PRA) and aldosterone in three groups of subjects. Group 1 consisted of seven Type I diabetics with microalbuminuria (greater than 25 micrograms/min), age 12.0-19.5 yr (mean +/- SD: 15.4 +/- 2.2), duration of disease 6.5-10.1 yr (7.9 +/- 1.9), HbA1c 9.6-16.0% (12.6 +/- 2.9). Group 2 consisted of seven sex and age-matched diabetics, duration of disease 3.7-9.0 yr (6.0 +/- 2.3), HbA1c less than 8%, microalbuminuria less than 10 micrograms/min, and microangiopathy-free. Group 3 consisted of seven healthy subjects. After overnight recumbency the PRA in group 1 patients was significantly higher than that for group 2 (3.926 +/- 4.54 ng/ml/h vs. 1.416 +/- 0.44; p less than 0.05) or for group 3 (3.926 +/- 4.54 vs. 1.11 +/- 0.82; p less than 0.007). After physical exercise the group 1 PRA value (10.199 +/- 9.62) was higher than in either group 2 (2.821 +/- 1.77; p less than 0.005) or group 3 (1.61 +/- 0.803; p less than 0.0006). Poor metabolic control and the presence of microalbuminuria can play a role in perturbations of the Renin-Angiotensin system. The presence of microalbuminuria can be an important indicator of mildly impaired renal function and may influence PRA production.