The stimulatory effects of 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin) on male sexual behavior in the absence and presence of testosterone were investigated. Male rats (n = 12) were castrated and tested for sexual behavior (15 min, with an estrous female) up to 1 year after castration. Castration caused an abrupt decrease (50%) in the number of intromissions before first ejaculation, and the number remained stable for about 8 weeks. Between 8-12 weeks after castration, when ejaculation frequency was low, 0.2 or 0.4 mg/kg 8-OH-DPAT had no effect on ejaculation frequency, but significantly decreased the number of intromissions before first ejaculation. In weeks 17-18 after castration, 0.4 mg/kg 8-OH-DPAT no longer affected copulatory behavior. A 5-mm testosterone-filled Silastic capsule (implanted at week 19 after castration) resulted in subnormal plasma testosterone levels (mean 4.4 nmol/l) and did not fully restore male copulatory behavior. Administration of 8-OH-DPAT (0.2 and 0.4 mg/kg) was followed by an increase in ejaculation frequency and a decrease in ejaculation latency. Five months later, when plasma testosterone levels were very low (mean 0.6 nmol/l), 8-OH-DPAT (0.4 mg/kg) significantly increased the mean number of intromissions and ejaculations and decreased the number of intromissions before first ejaculation, intromission latency and ejaculation latency (borderline). The present results suggest that testosterone is required for the activating effects of 8-OH-DPAT on sexual behavior in castrated male rats, tested 17-52 weeks after castration.