Socal recognition of juvenile conspecifics by adult male rats has been shown to be modulated by vasopressin. Because part of the extrahypothalamic vasopressinergic innervation of the brain is androgen-dependent, the present experiments were undertaken to assess possible interactions between androgens and vasopressin in social recognition. Castrated male rats displayed a temporary disruption of social recognition when they were tested 1 week after surgery. There was no impairment, however, when they were tested every other day following surgery. The peripheral injection of a vasopressor antagonist of vasopressin, dPTyr(Me)AVP (30 micrograms/kg) impaired social recognition in normal male rats but was ineffective in castrated ones. This was not due to an effect of castration on the basic pharmacological properties of dPTyr(Me)AVP since the antagonist peptide was still able to block the facilitating effects of vasopressin on social recognition in castrated male rats. Implantation of a testosterone-filled capsule in castrated male rats restored sensitivity of social recognition to the action of the vasopressin antagonist. These results suggest that androgen-dependent vasopressinergic neurons are physiologically involved in the modulation of social recognition in male rats.