The renal actions of atriopeptin III were examined in anaesthetised rats at differing perfusion pressures before and following blockade of the renin-angiotensin system. At normal perfusion pressure 1000 ng kg-1 atriopeptin III caused reversible increases in glomerular filtration rate, of 20%, urine flow, absolute and fractional sodium excretions of 51-93%. Reduction of left renal perfusion pressure to 80 mm Hg decreased glomerular filtration rate by 30% and urine flow, absolute and fractional sodium excretions by 80% while atriopeptin III administration only minimally changed these variables. Concomitantly, right kidney perfusion pressure rose by 15 mm Hg which significantly increased fluid output, while the atriopeptin III induced diuresis and natriuresis were significantly larger. During infusion of captopril 900 micrograms kg-1 h-1 when pressures at the left and right kidneys had been reduced and elevated, respectively, atriopeptin III caused larger excretory responses in both kidneys which were greater than without captopril. These result suggested that the atriopeptin III mediated natriuresis and diuresis were directly proportional to perfusion pressure and attenuated by angiotensin II.