In the male rat, subcutaneous injections for 7 days of 20 mg/Kg B.W./day of 5-hydroxytryptamine creatinin sulphate (5-HT), caused remarkable inhibitory effects on sexual behavior. The mount and intromission latencies were increased in rats treated with 5-HT, whereas ejaculation latency in the few rats treated with 5-HT that it achieved, was similar to that obtained in control rats. The mount and intromission frequencies were decreased in the rats treated with 5-HT. The mean inter-intromission interval (MII) and post-ejaculatory interval were prolonged in rats treated with 5-HT. These data provide evidence for the role of peripheral 5-HT in regulating sexual behavior of - male rats. © 1982.