Acute and chronic effects of the converting enzyme inhibitor lisinopril on renal and systemic hemodynamics were studied in 12 patients with mild to moderate essential hypertension. After a washout period, cardiac output (measured by Doppler echography), renal plasma flow, and glomerular filtration rate (measured by isotopic techniques) were determined before and after oral administration of 20 mg lisinopril (visit 1). The same protocol was repeated after 3 months of lisinopril therapy 20 mg once daily (visit 2). Acute administration of lisinopril, both in untreated hypertensive patients (visit 1) and during long-term treatment (visit 2), decreased blood pressure (p < 0.05) and increased renal plasma flow (p < 0.05), while cardiac output and glomerular filtration rate were unchanged. Comparison of baseline parameters between visits 1 and 2 showed that chronic treatment with lisinopril decreased blood pressure and renal vascular resistance and that these effects were still significant 24-hours postdosage. We conclude that lisinopril is an effective antihypertensive agent with favorable renal hemodynamic effects.