Penile erection and ejaculation are produced by spinal reflexes subject to tonic control from the brain. This study examines the possible involvement of serotonergic transmission in the supraspinal modulation of such reflexes. The effects of two drugs which facilitate serotonergic transmission by different mechanisms, namely the direct receptor agonist, 5-methoxy-N,N'-dimethyltryptamine (5-MeODMT), and the reuptake inhibitor, zimelidine, were compared in intact and spinal rats. Results show that serotonergic stimulation in intact rats by either drug produces a dose-related increase in the incidence of seminal emission as well as a definite decrease of the display of erectile responses. In the spinal animals 5-MeODMT treatment reproduced both effects. By contrast, zimelidine, which needs functional nerve endings to exert its agonistic action, was ineffective in the spinal rats. This is interpreted to exclude a peripheral mechanism for the effects of the serotonin agonists on penile reflexes of intact animals and makes a strong case for a spinal site of action. We postulate the existence of serotonergic receptors located in the lower segments of the spinal cord which, when stimulated, trigger seminal emission and suppress erection.