Abstract. The acute effects of physiological levels of AVP and oxytocin administration on renal water and sodium handling have been investigated in New Zealand genetically hypertensive and normotensive rats. AVP infusion was associated with an antidiuresis in both normotensive and hypertensive rats and while normotensive rats also displayed a dose-related natriuresis, this was attenuated in hypertensive rats. Oxytocin administration had no effect on urine flow or sodium excretion in normotensive rats, but was associated with an antidiuresis in hypertensive rats. Combined hormone infusion produced a greater reduction in urine flow than following AVP alone in both normotensive and hypertensive groups and was associated with a potentiation of the natriuretic action of AVP in the hypertensive animals. The data suggest that the contribution of oxytocin to renal sodium excretion in hypertensive rats may be suppressed. A compensatory increase in basal AVP secretion in hypertensive rats may overcome their apparent renal insensitivity to AVP, to maintain appropriate sodium excretion. This intriguing disturbance in neurohypophysial function may reflect or possibly contribute to the hypertension of these animals.