1. The present study was carried out to investigate the haemodynamic and natriuretic effects of atrial natriuretic peptide (ANP) in normal rats and in rats with unilateral ischaemia.

2. Twenty-four hour unilateral ureter occlusion gave rise to a marked vasoconstriction in the ipsilateral kidney after its release. Intravenous infusion of ANP doubled p-aminohippurate clearance and inulin clearance and elicited massive natriuresis in the hydronephrotic kidney, while in the contralateral kidney these clearance values were decreased and there was a lack of natriuresis. The responses in the latter kidney were also different from those in the normal rat, in which significant natriuresis was elicited by ANP even though a decrease in p-aminohippurate clearance occurred.

3. After the control kidney in the unilateral hydronephrotic rat was denervated either mechanically or by pretreatment with prazosin, ANP induced a natriuresis in both kidneys. Furthermore, the renal denervation prevented the decrease in inulin clearance in the control kidney after ANP administration.

4. These findings suggest that the renal response to ANP may depend on the vascular tone before administration, and that renal nerve activity may modify the effects of ANP on renal haemodynamics and sodium excretion.

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