We have proposed that the organum vasculosum laminae terminalis (OVLT) contains osmoreceptors involved in the regulation of drinking and secretion of arginine vasopressin (AVP) in the dog. By use of the technique of electrolytic ablation, conscious dogs were evaluated for their responses to acute peripheral challenges of hypertonic NaCl and angiotensin II (ANG II) and hemorrhage (20 ml/kg) before and after destruction of the OVLT (n = 7) or adjacent tissue (n = 9) as a control. Lesions confined to the tip of the optic recess and destroying greater than 90% of the OVLT caused a dramatic increase in the threshold to drink and increase plasma AVP levels and a significant (P less than 0.01) reduction in the magnitude of these responses to an osmotic stimulus. Drinking and secretion of AVP in response to ANG II, but not the rise in peripheral corticosteroids, were also blocked (P less than 0.01) by ablation of the OVLT. In contrast responses to hemorrhage were not affected by the lesion. These results support the hypothesis that the OVLT contains osmoreceptors involved in physiological responses to cellular dehydration and suggest involvement in drinking and secretion of AVP in response to ANG II. The results also indicate that forebrain mechanisms which depend on the OVLT are independent of mechanisms responding to hemorrhage in conscious dogs.