We used neuronal nitric oxide synthase (nNOS) gene knockout mice to study the effects of catecholamines and neuronal nitric oxide on vasopressin expression in the hypothalamic neurosecretory centers. nNOS gene deletion did not change the level of vasopressin mRNA in the supraoptic or paraventricular nuclei. In contrast, vasopressin immunoreactivity was lower in nNOS deficient mice than in wild-type animals. Dehydration increased vasopressin mRNA levels and decreased vasopressin immunoreactivity in both wild-type and nNOS knockout mice, but these responses were more marked in the nNOS knockout mice. Treatment with alpha-mpt, a pharmacologic inhibitor of catecholamine synthesis, resulted in increased vasopressin mRNA levels in wild-type mice and in reduced vasopressin immunoreactivity in both wild-type and nNOS knockout mice. From these results, we conclude: (1) neuronal nitric oxide suppresses vasopressin expression under basal conditions and during activation of the vasopressinergic system by dehydration; (2) catecholamines limit vasopressin expression; (3) nNOS is required for the effects of catecholamines on vasopressin expression.