The role of the accessory olfactory bulb in conveying pheromonal information to tuberoinfundibular arcuate neurons was examined electrophysiologically in chloral hydrate-anaesthetized, oestrogen (0.5 micrograms in silastic capsules)-treated and untreated ovariectomized Balb/c female mice. Electrical stimulation of the accessory olfactory bulb orthodromically excited part of tuberoinfundibular neurons which were antidromically stimulated from the median eminence and histologically verified as being located within the arcuate nucleus. No inhibitions followed accessory bulb stimulation. The excitatory response to accessory bulb stimulation was reversibly blocked by the local anaesthetic lignocaine infused into the amygdala. The percentage of tuberoinfundibular arcuate neurons responding to accessory bulb stimulation was significantly higher in oestrogen-treated than in untreated animals. There was no difference between the two groups for the antidromic activation threshold, spontaneous firing rate, absolute refractory period or frequency of successful antidromic propagation into the soma of tuberoinfundibular arcuate neurons. In oestrogen-treated preparations, tuberoinfundibular arcuate neurons responsive and unresponsive to accessory bulb stimulation could be distinguished by the frequency of successful antidromic propagation into the soma. These studies demonstrate that olfactory relay neurons in the accessory olfactory bulb act to enhance the activity of a subpopulation of tuberoinfundibular arcuate neurons via the amygdala and that this neural transmission is modulated by oestrogen.