Injection of 5,6-dihydroxytryptamine (5,6-DHT) into the lateral ventricles of rats causes a marked increase in copulatory activity concomitant with a rather selective decrease of cerebral 5-hydroxytryptamine (5HT). 5-Hydroxytryptophan given i.p. counteracts both effects of 5,6-DHT. I.v. injection of 6-hydroxydopamine prior to 5,6-DHT causes a considerable decrease of brain catecholamines without enhancing the diminution of 5HT and, moreover, does not antagonize the copulatory activity. These results indicate that the 5,6-DHT-induced stimulation of copulatory behaviour in rats is correlated with a diminution in brain 5HT.