Although angiotensin-converting enzyme (ACE) inhibitors may lower urinary protein excretion, it is not known whether these agents can completely eliminate microalbuminuria. This study examined whether the ACE inhibitor, enalapril, can abolish low levels of microalbuminuria in diabetic patients. Six men with adult-onset, insulin-dependent diabetes mellitus, most of whom had low levels of microalbuminuria, were studied in a clinical research center, where they ate a controlled diet and performed regulated exercises daily. After 2 weeks of baseline measurements, the patients received 5-15 mg/day of enalapril for 4 weeks. They were then monitored for 2 more weeks without enalapril. Urinary albumin excretion (UAE) fell in each patient with enalapril treatment and was within the normal range at some time during enalapril treatment in 5 of 6 patients. After stopping enalapril, UAE rose. UAE was 53.6 +/- 20.7 (SEM), 31.5 +/- 8.9 and 39.4 +/- 8.0 mg/24 h during the baseline, enalapril and postenalapril periods, respectively (baseline vs. enalapril, p less than 0.02; postenalapril vs. enalapril, p less than 0.01). The magnitude of fall in UAE correlated with the baseline UAE (r = 0.90). During enalapril treatment, renal plasma flow and GFR did not change, although blood pressure fell slightly. These data suggest that enalapril can reduce or abolish UAE in diabetic patients with microalbuminuria. Whether long-term treatment with enalapril will continue to suppress microalbuminuria and prevent progressive diabetic nephropathy remains to be determined.