Renal haemodynamics and natriuresis were studied before and 6 h after oral intake of perindopril (8 mg) in eight hypertensive patients without renal failure. The patients were then treated with perindopril (8 mg per day) and renal haemodynamics were measured on the fifth day, 6 h after the morning intake. Sodium intake was controlled during the study (100 mmol sodium per day). Renal blood flow and the glomerular filtration rate were measured by the clearance method using 131I-hippuran and 125I-iothalamate, respectively. Mean blood pressure decreased from 135 to 110 after 6 h, and was 118 mmHg on the fifth day (P less than 0.001, respectively). Renal vascular resistance decreased significantly after acute drug intake from 0.19 to 0.15 arbitrary units (P less than 0.001) and on the fifth day to 0.16 arbitrary units (P less than 0.001). Renal blood flow rose from 708 to 723 after 6 h, and to 750 ml/min per 1.73 m2 on the fifth day but the change was no significant. There was no alteration in the glomerular filtration rate so that the filtration fraction decreased from 0.27 to 0.26 (after 6 h), and to 0.25 on the fifth day (P less than 0.02). Natriuresis increased after the first intake between the first and tenth hours. On the fifth day, maximum natriuresis was observed between the fourth and sixth hours. Perindopril caused strong renal vasodilation after the first intake and during the following days, with no change in the glomerular filtration rate. There was a significant decrease in the filtration fraction, indicating efferent, as well as afferent, arteriolar vasodilation.(ABSTRACT TRUNCATED AT 250 WORDS)