Nineteen essential hypertensive patients were entered into a protocol to assess the BP, humoral and renal effects of the angiotensin converting enzyme inhibitor, lisinopril (MK 521, 20 to 80 mg once daily), administered for 52 weeks. Specifically monitored prior to, and following 12 and 52 weeks of lisinopril monotherapy were plasma renin activity and plasma aldosterone, the clearances of creatinine, inulin and para-aminohippurate, and the 24-hour urinary excretion of protein. BP was well controlled. Plasma renin activity was stimulated, and plasma aldosterone was suppressed throughout the entire protocol. In contrast to the reported short-term and long-term renal effects of enalapril, lisinopril (a lysine analog of enalapril) had no short-term effect on renal function: glomerular filtration rate, effective renal plasma flow, filtration fraction (FF), renal vascular resistance (RVR), and protein excretion were all unchanged. However, following long-term therapy, both FF and RVR were decreased. Lisinopril appears to convey no specific renal pharmacological benefit.