There is an increasing amount of evidence, from several mammalian species, that plasma testosterone levels increase acutely when the male is exposed to sexual stimuli. Work reported here investigates whether such responses occur in male mice, and whether their incidence is related to measures of sexual behaviour. Close proximity of oestrous female mice was found to result in increased plasma testosterone levels in some strains within 15 min. Immediately after a sexual behaviour test, plasma testosterone levels were higher in males showing sexual responses. There were correlations between testosterone levels and mount latency, but none with ejaculation latency. Testosterone levels were greatest at the initiation of mounting responses, and declined during copulation, although not significantly.