An abundance of experimental literature has established that gonadal steroid hormones are responsible for the sexual differentiation of neural circuitry, mediating a rarity of reproductive behaviors and physiological mechanisms. These same hormones regulate the expression of reproduetive function in the adult and may influence the responsiveness of the brain to specific olfactory cues. The recent demonstration that the expression of the neuropeptide cholecystokinin is activationally regulated by estrogen at the mRNA level, within a sexually dimorphic p@ ulation of neurons in the medial amygdala, suggests a possible cellular mechanism for the hormonal modulation of olfactory information relayed along the vomeronasal pathway to the h39othalamus.