SUMMARYIn the absence of the potentially confounding influence of vasopressin in hypertension, the effects of atrial natriuretic peptide (ANP) on arterial blood pressure and renal handling of water and sodium were assessed from comparison of responses to intravenous ANP infusion in anaesthetized vasopressin-deficient New Zealand genetically hypertensive (DI/H) rats and their normotensive substrain (DI/N). After 320 min of hypotonic saline infusion, plasma ANP concentration was significantly higher in DI/H compared with DI/N rats. ANP administration increased circulating ANP concentration in both groups. Plasma angiotensin II concentration was higher in DI/H than in DI/N rats; infusion of ANP raised circulating angiotensin II in both groups though this achieved statistical significance only in DI/N rats. Plasma aldosterone concentrations were initially similar in normotensive and hypertensive animals and, in both, were reduced markedly by i.v. ANP infusion. Administration of ANP produced sustained hypotension in both groups. However, the hypotensive effect of ANP was more pronounced in DI/H compared with DI/N rats. Heart rate was initially similar in the two groups, and infusion of ANP produced no detectable change. By comparison with animals maintained on hormone-free infusate, urine flow increased markedly over the 80 min period of ANP infusion in both groups, by 142 % in DI/H rats and 127 % in DI/N rats. ANP administration produced a natriuresis in both groups but the increase in Na+ excretion was much greater in DI/H (342 %) than in DI/N (139 %) rats. It appears from the current study that vasopressin-deficient hypertensive rats are more sensitive to ANP with regard to effects on blood pressure and renal excretion than their vasopressin-deficient normotensive substrain. These differences may, in part, reflect adjustments to long-term elevation in blood pressure and in plasma ANP concentration in hypertension and, in part, rely on the associated disturbances in related endocrine systems.