Conscious two-kidney, one-clip hypertensive rats were chronically infused during 7 days with synthetic ANF (8-33) (1 yg/ hr/rat) by means of osmotic minipumps. The initial blood pressure of 183 ± 4 mmHg gradually decreased to 116 ± 5 mmHg the last 2 days of infusion. Pressure diuresis returned to normal and pressure natriuresis was attenuated. PRA was significantly lower (1.81 ± 0.41 AI ng/ml/hr) than in not treated hypertensive rats (8.56 ± 3.75 AI ng/ml/hr). A partial regression in cardiac hypertrophy was observed in the treated group. We suggest that the hypotensive response to ANF may be mainly due to vasodilatation, but the possibility that the decrease in PRA may play a partial role in lowering blood pressure, cannot be excluded.