Female mice which have mated and are subsequently exposed to the odour (pheromones) of a strange male undergo hormonal changes resulting in a block to their pregnancy. The fact that the stud male's odours can also block pregnancies, that is other than his own, implies the formation of a memory or some form of recognition process by the female for this male's pheromones at the time of mating. The purpose of this study was to evaluate the effect of microinfusions of drugs which interfere with neural transmission, into the accessory olfactory bulbs. This was carried out immediately after mating over a 4-h period during which the "memory" to the stud male's pheromones is formed. Infusions of the alpha-blocker, phentolamine, blocked the formation of the olfactory memory, while the GABA receptor blocker, bicuculline, itself blocked pregnancy, but was without effect on memory formation. Protein synthesis inhibition or calpain inactivation in the accessory bulb was without effect on memory formation at any of the doses used. These studies demonstrate that GABAergic transmitter blockade in the accessory olfactory bulb at the time of mating can prevent subsequent blastocyst implantation some 3 days later, while alpha-noradrenergic blockade can prevent the formation of an olfactory memory to the stud male.