Several lines of evidence suggest that nitric oxide (NO) is involved in the neuroendocrine control of reproductive function. This study was undertaken to determine 1) NO activity in the medial preoptic area (MPOA) where LHRH- and NO synthase-containing neurons are coextensive; 2) whether N-methyl-D-aspartate (NMDA) receptor activation, which stimulates LHRH release, augments NO activity in the MPOA; and 3) whether NO activation in the MPOA underlies the steroid dependency of NMDA-induced pituitary LH release. As extracellular levels of cGMP in discrete brain sites are a reliable index of basal and stimulated activity of NO, extracellular cGMP levels in the MPOA of freely moving, awake rats were measured by microdialysis in the current study. In the first experiment, the MPOA of intact and castrated male rats were microdialyzed with artificial cerebrospinal fluid at a rate of 5 microliters/min. The basal level of cGMP efflux was determined from the initial seven samples collected at 20-min intervals. The NO response to a single i.v. injection of NMDA (10 mg/kg) or saline was assessed in the next five samples. In the second experiment, the basal and NMDA-evoked NO effluxes in the MPOA of ovariectomized (ovx) and estrogen-treated ovx rats were examined. Results showed that in both sexes, the absence of gonadal steroids resulted in significantly lower basal cGMP levels. Additionally, the cGMP response to NMDA was steroid dependent. Whereas in castrated rats it failed to affect cGMP efflux, NMDA in intact male rats promptly raised cGMP levels at 20 min, and these elevated levels were maintained through the duration of the experiment. This NMDA-induced cGMP response, observed selectively in intact rats, was also associated with stimulation of plasma LH levels. In female rats, NMDA similarly enhanced MPOA cGMP efflux and pituitary LH secretion in estradiol benzoate-treated, but not in oil-treated, ovx rats. The NMDA receptor antagonist D,L-amino-5-phosphoropentanoic acid and the NO synthase inhibitor, N omega-nitro-L-arginine, completely blocked the NMDA-induced cGMP and LH responses, thereby demonstrating the specificity of the NMDA receptor —> NO line of communication probably operating in the MPOA in the control of pituitary LH release. Therefore, these results show that gonadal steroids augment basal as well as NMDA-induced MPOA cGMP efflux in male and female rats. It is likely that facilitation of NO/cGMP activity in the MPOA may underlie the steroid dependency of NMDA-evoked LH hypersecretion in the rat.