Male hamsters (intact, castrated or testosterone-treated) were tested for their response to dioestrous and to oestrous females. Response to the dioestrous female was measured in terms of proximity and olfactory investigation; response to the oestrous female was measured in terms of mounting, intromission and ejaculation. Interest in dioestrous females declined in parallel with copulatory behaviour after castration and was restored by implantation of testosterone. Six weeks of testosterone treatment failed to restore copulatory behaviour to pre-castration levels; in contrast, interest in the dioestrous female was fully restored. Different components of the behaviour towards dioestrous female changed at different rates; some were restored within a week of implantation while others took several weeks to reach pre-castration levels. As found previously, the tendencies of the male to approach and to leave the female varied independently.