Vasopressin binding sites were detected in the brain of the Siberian hamster, using [3H]vasopressin and a 125I-labelled linear vasopressin antagonist specific for V1 vasopressin receptors. In the ventromedial and premammillary nuclei, the density of the binding was lower in the females than in the males. The effect of castration and of testosterone replacement was assessed in males. Two distinct effects were observed. Orchidectomy diminished significantly the vasopressin binding in the ventromedial nucleus, an effect which was prevented by implantation of a mini-pump releasing testosterone. On the contrary, in the premammillary nucleus no significant differences were noticed following castration and testosterone treatment. In addition, vasopressin immunoreactivity was examined in males, in females and in castrated males. No sex differences were evident. However, in the bed nucleus of the stria terminalis and the lateral septal nucleus, castration decreased vasopressin immunoreactivity in either sex. This effect of castration was prevented by testosterone. Vasopressin immunoreactivity was detected neither in the ventromedial nor in the premammillary hypothalamic nuclei. Our observations suggest that, in adult Siberian hamster premammillary nucleus, the expression of vasopressin receptors is not controlled by gonadal steroids but is sex related and could be induced during fetal or early postnatal life.