The most important component of the renal-body fluid feedback is renal pressure natriuresis which through sodium and water excretion stabilizes arterial pressure. The chronic effects of angiotensin II in regulating the pressure natriuresis was studied in dogs by the Guyton school. Renal perfusion was either permitted to increase or was maintained constant with a servo-controlled occluder placed on the abdominal aorta just above the kidneys. When the renal pressure was allowed to increase, sodium excretion was reduced for a day and after 4-5 days there was little net change in sodium balance and arterial pressure stabilized at about 30 mm Hg above control. In the servo-controlled renal perfusion, escape from sodium retention did not occur, arterial pressure continued to rise, and pulmonary edema developed. Angiotensin II, by its hemodynamic and tubular effects, modulates renal sodium and water excretion and has an important role in blood pressure regulation. Antibodies to renin and converting enzyme inhibitors showed a causal relationship between the stimulated renin-angiotensin system and the antihypertensive effect of these agents. Chronic effects are observed in hypertensive patients with normal or even low plasma renin activity. This suggests that local angiotensin concentrations in the vascular and renin tissues may be more important in determining sodium and water excretion. Our knowledge of the renin-angiotensin system in regulating blood pressure made the usage of converting enzyme inhibitors a logical and efficacious modality in the therapy of hypertension. In a multicenter study of 202 hypertensive patients, the efficacy and safety of ramipril and enalapril was studied.(ABSTRACT TRUNCATED AT 250 WORDS)