Castration in rats of either sex has been shown to markedly decrease hypothalamic oxytocin binding in the ventromedial hypothalamus and this can be reversed by injecting gonadal steroids. We wondered whether castration exerts similar effects on homologous oxytocin binding sites present in the guinea pig hypothalamus. Adult male guinea pigs were castrated and killed 2-90 days later. Binding sites for oxytocin in the ventromedial nucleus and neuronal responses to this peptide were little affected by gonadectomy, in contrast to what is observed in the rat under similar experimental conditions. The steroid dependency of hypothalamic oxytocin receptors appears therefore to be species dependent.