Single unit activity was recorded from neurones in the preoptic and anterior hypothalamic areas of normal male and female rats as well as neonatally castrated males and females neonatally treated with testosterone propionate. The neurones were initially classified according to their responses to single shock stimulation of the mediobasal hypothalamus and subsequently according to their response following stimulation of the corticomedial amygdala. Cells were either antidromically invaded, orthodromically activated or inhibited, or unaffected by the stimulus. All possible response combinations were observed. Thus neurones in the preoptic area project directly to the region of the median eminence and to the amygdala; they also receive excitatory and inhibitory afferent fibres from both these regions. Evidence is provided for a substantial convergence of either excitatory or inhibitory inputs onto many of these cells. The majority of response combinations showed no dependence on the sex of the animal. However, significantly more of the cells projecting to the mediobasal hypothalamus receive synaptic connections from the amygdala in males than in either normal females or neonatally castrated males. Females neonatally treated with testosterone occupied an intermediate position. Very few of the cells were found in that part of the preoptic area previously shown to be sexually dimorphic. Neurones which were not influenced by stimulation of the mediobasal hypothalamus fired twice as fast in normal females and neonatally castrated males as the same cell type in normal males and females neonatally treated with testosterone. Thus the sex dependent properties of two categories of neurone are related to endocrine status and not genetic sex, the male situation being induced by exposure of the brain to androgen during the critical perinatal period.