THE odours of adult males, which accelerate the timing of puberty of female mice, activate c-fosin the accessory olfactory bulb (AOB). To test the hypothesis that NMDA receptors are involved in the male odour-induced increase in c-fosexpression, we studied the effects of the non-competitive NMDA receptor agonist MK-801 on male odour-induced c-fosexpression in the AOB of juvenile female mice. Surprisingly, MK-801 increased FOS-like immunoreactivity (FLI) within the AOB in the absence of male odour and had no effect on male odour-induced c-fosexpression. We suggest that MK-801 increases AOB mitral cell activity by disinhibiting GABAergic granule cells, resulting in increased c-fosexpression throughout the AOB.