The acute haemodynamic effect of perindoprilat was examined at rest, supine and sitting, and during 100W bicycle exercise in 12 patients (mean age 42 years) with essential hypertension. Intra-arterial blood pressure and the heart rate were recorded continuously. Cardiac output was measured by dye dilution (Cardiogreen) and blood volume was determined by radio-iodinated (125I) human serum albumin. Two hours after a slow (3min) intravenous injection of perindoprilat, blood pressure was reduced in all patients (P<0.01) — at rest sitting from 175/108 to 153/97 mmHg (11%) — because of reduction in total peripheral resistance index (f=2.63; P<0.05). Only minor changes were seen in the heart rate, stroke index and cardiac output. The fall in blood pressure was significantly (P<0.05) correlated with blood volume (r=0.65) and pretreatment total peripheral resistance index (r=0.59).