1. This study was designed to investigate the effects of intracerebroventricular or intravenous atrial natriuretic peptide (ANP) on plasma arginine-vasopressin (AVP) levels, plasma renin activity (PRA), blood pressure (BP) and heart rate (HR) in conscious rats.

2. No changes were observed in plasma AVP levels and PRA after intracerebroventricular injection of ANP (0.3 nmol/kg). Also no changes were found in BP and HR after intracerebroventricular ANP injection in the dose range 0.01–0.3 nmol/ kg.

3. No significant changes were observed in plasma AVP levels after the intravenous administration of ANP at 2.5 nmol/kg to hydrated rats or at 0.1 nmol/kg and 2.5 nmol/kg to dehydrated rats, although the larger dose was sufficient to cause a small fall in BP.

4. Even after 30 min intravenous infusion of ANP at 0.1 nmol min−1 kg−1, a dose sufficient to produce very high plasma ANP levels, no change in plasma AVP could be detected, although haemodynamic changes were observed.

5. Intravenous ANP injection (2.5 nmol/kg) inhibited basal renin secretion in hydrated rats and also inhibited renin secretion which had been stimulated by prior dehydration.

6. From these studies in hydrated and dehydrated rats, there was no evidence that either intracerebroventricular or intravenous ANP administration affected AVP release in vivo.

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