To investigate the effects of antihypertensive treatment with the angiotensin-converting enzyme (ACE) inhibitor lisinopril on insulin sensitivity and related metabolic variables, the insulin sensitivity index (SI), determined with the Minimal Model Method of Bergman, fasting plasma insulin and glucose concentrations, serum total triglyceride and lipoprotein cholesterol fractions, and blood pressure were assessed in 24 lean, non-diabetic patients with essential hypertension. Following a double-blind, randomised crossover design, these parameters were measured after a 4-week run-in period, after 8 weeks of lisinopril or placebo, and after an additional 8 weeks on placebo or lisinopril, respectively. Furthermore, the level of physical fitness was estimated using the Conconi bicycle ergometer test. SI was low in this study population (5.6 vs 13.3 x 10(-4).min-1.mU-1.l-1 in normal lean control subjects). It did not differ between the placebo run-in phase, the lisinopril phase, and the placebo crossover phase (5.8, 5.5, and 5.4 x 10(-4).min-1.mU-1.l-1, respectively). Moreover, during the administration of lisinopril, no significant changes occurred in fasting plasma insulin and glucose, areas under the glucose and insulin curves, glucose disappearance rate, serum total triglycerides, and cholesterol or lipoprotein cholesterol fractions. Heart rate at rest, body weight, and anaerobic threshold remained stable throughout the study. Compliance assessed by pill-counting exceeded 90% at all visits. These findings demonstrate that the ACE inhibitor lisinopril is neutral with regard to insulin sensitivity, plasma insulin and glucose, and lipoprotein metabolism in patients with essential hypertension.