The effect of enalapril on albumin excretion rate was studied in two groups of age-and sex-matched Type 1 (insulin-dependent) diabetic patients, aged 15-20 years, with persistent microalbuminuria > 20 gg/min. Group i contained six patients with systolic blood pressure > 75th percentile for age and sex, group 2 six normotensive patients. Enalapril (10-20 mg/day) was given for six months. Albumin excretion rate, glomerular filtration rate, renal plasma flow, blood pressure at rest and during exercise, and angiotensin converting enzyme activity were measured before, after three weeks' and six months' treatment and six months after treatment withdrawal. Albumin excretion rate decreased in all patients after three weeks' (mean decreases 55% in group 1, 65% in group 2) and six months' treatment (35% in group 1, 61% in group 2). Systolic blood pressure remained unchanged in both groups. Diastolic pressure was reduced after three weeks in group 1 (p = 0.001). No reduction in increment in systolic pressure during exercise test occurred in any group during treatment. Angiotensin converting enzyme activity decreased in all patients after three weeks (p = 0.001) and six months (p = 0.003). This correlated to the decrease in albumin excretion rate after three weeks (r --0.79,p = 0.05) and six months (r = 0.59, p = 0.04). HbAI~,, mean blood glucose and glomerular filtration rate remained unchanged during the study in both groups. Renal plasma flow tended to increase after three weeks' and six months' treatment in group 2 (p = 0.06, respectively) but not in group 1. Filtration fraction decreased after three weeks (p = 0.04) only in group 2. In conclusion, enalapril reduces the albumin excretion rate in adolescent diabetic patients with or without elevated blood pressure. This reduction was not accompanied by a decreased systemic pressure but rather by a fall in filtration fraction in normotensive patients, indicating a direct effect, irrespective of the antihypertensive, on intraglomerular pressure.