The distribution of immunoreactivity to neuronal nitric oxide synthase (nNOS) and vasopressin (AVP) was studied in the circumventricular organs of the female rat. The occurrence of NOS immunoreactivity showed correspondence to nicotinamide dinucleotide phosphate diaphorase reactivity, a previously used but less specific marker for neuronal NOS. nNOS immunolabeling was detected in the two most rostrally located circumventricular organs - the organum vasculosum of the lamina terminalis and the subfornical organ. In the latter, AVP immunoreactivity was observed in some cell bodies, which also were nNOS-immunoreactive. In the median eminence and the neurohypophysis there were large amounts of nNOS- and AVP-immunoreactive nerve fibers, which often displayed similarities in distribution and morphology. Within the pineal gland, only very few nNOS-immunoreactive varicose terminals were observed, which ran along blood vessels. nNOS immunoreactivity was also seen in the epithelium of the choroid plexus, whereas no nNOS immunoreactivity could be found in the subcommissural organ or in the area postrema. The present demonstration of nNOS and AVP immunoreactivity in the subfornical organ, median eminence, and neurohypophysis, and the occurrence of nNOS immunoreactivity also in the choroid plexus and organum vasculosum of the lamina terminalis, provides a morphological background for a functional role for nitric oxide in water homeostatic mechanisms, both as executed through the hypothalamohypophyseal system and via the production of cerebrospinal fluid.