The magnocellular medial preoptic nucleus (MPN mag), has been implicated in male sex behavior, presumably by mediating pheromonal induction of copulation in the hamster. The nucleus is sexually dimorphic; only the male MPN mag shows stimulation following exposure to pheromones. The mechanism that underlies this dimorphism is unknown.  Nitric oxide (NO) has been shown to mediate the expression of male sexual behavior induced by pheromones.  The current study used a stereological approach, in order to determine if there is a sex difference in Nitric Oxide Synthase (NOS), the enzyme that creates NO. The MPN mag of males contained more NOS labeled puncta than females, while the number of labeled neurons containing NOS was not different between the sexes. Double labeling with antibodies to PSD 95 and neurofilament (only found in axons)  suggests that the NOS puncta lie in the postsynaptic structures.  In light of the role of NOS in copulation and the location of sexually dimorphic NOS populations in neuronal groups that project to the MPN mag, the results suggest that the differences in NOS in the MPN mag may underlie the sex differences in response to pheromones in the hamster.