1. The endothlins (ETs) are a family of homologous peptides originally isolated and purified from cultured porcine endothelial cells. Although initial studies focused on the ETs as potent vasoconstrictor substances, recent findings support a role for ET in vasopressin (AVP) secretion and action. We used cultured explants of the hypothalamo-neurohypophysial complex (HNC) to examine the effects of ET-3 on AVP release. 2. ET-3 produced a significant and concentration-dependent rise in AVP release from the explants of Long-Evans rats at 48 h, independent of medium osmolality. AVP release during two sequential control periods did not differ, and osmotically stimulated AVP release was comparable to that exhibited by HNC explants from Sprague-Dawley rats. ET-3 (1 nM) induced a 3-fold rise in AVP that was completely blocked by rabbit antiET serum, which did not alter basal AVP secretion. 3. Clearance experiments were performed in anaesthetized water-loaded rats given a non-pressor dose of ET-3 (0.40 nmol (kg body weight)-1 intravenously). The means of body weight, arterial blood pressure, plasma Na+ concentrations, and plasma osmolalities did not differ between the first and second periods nor among the groups. Despite no changes in renal plasma flow and inulin clearance, free water clearance significantly increased in the ET-treated group. This response could not be attributed to changes in osmolal clearances or Na+ reabsorption. The concurrent administration of antiET serum not only blocked this effect, but was associated with the free water clearance falling significantly below the pretreatment level. 4. Taken together, our in vivo and in vitro findings support the hypothesis that ET-3 stimulates AVP secretion. Furthermore, our data are consistent with the site of action of ET-3 residing within the blood-brain barrier, though an independent effect by a higher concentration of ET at neural loci outside the barrier cannot be totally excluded. Finally, the subpressor dose of ET-3 amplifies free water excretion independent of systemic and renal haemodynamics, Na+ excretion, osmolal clearance, or circulating AVP levels.