The effect on blood pressure (BP) of acute and chronic suppression of angiotensin (ANG) II was studied in the two-kidney, one clip hypertensive rat. Conscious, chronically catheterized rats were given a bolus injection of captopril (2.5 mg/kg) followed by a chronic infusion of either dextrose or captopril (1 mg/kg per h) lasting 5 days. Blood pressure was measured continuously by a computer technique. Following the acute injection of captopril, arterial BP fell from 165.1 +/- 19.4 mmHg (mean +/- s.d.) to a minimum of 137.6 +/- 23.3 mmHg after 15 min. Twelve hours after starting the chronic infusion of captopril, BP fell to a minimum of 112.5 +/- 19.4 mmHg. This was significantly lower than that after the acute injection of captopril. Blood pressure remained lower throughout the 5-day infusion ranging, on the 5th day, from 122.1 +/- 23.4 to 136.0 +/- 30.2 mmHg. In contrast, BP continued to rise in rats given dextrose chronically ranging, on the 5th day, from 163.6 +/- 23.8 to 180.4 +/- 22.5 mmHg. Both the fall in pressure after acute captopril and that after chronic captopril were related to pre-treatment levels of plasma renin concentration. These results suggest that in the two-kidney, one clip hypertensive rat ANG II, in addition to its acute vasoconstrictor property, contribute to the hypertension through a secondary effect, the mechanism of which is as yet uncertain.