To clarify the role of serotonin in penile erection, testosterone-primed castrated male rats were treated with the serotonin-synthesis inhibitor, p-chlorophenylalanine (pCPA), and reflexive erection (RE; male supine, penile sheath retracted) and noncontact erection (NCE; penile erection evoked by remote sexual stimuli) tests were performed. Half the males were injected with 100 mg/kg pCPA 4 times before each test; control males were treated with saline instead of pCPA. In the RE test, compared to the control group, pCPA-treated males had a shorter erection latency, but they also displayed fewer erections. NCE tests were conducted as a 2 x 2 factorial experiment: pCPA or saline, and estrous female present or absent. Only the pCPA-female Group had a high proportion of responders (68%), compared to 14-27% in the other Groups (p < 0.02). These results suggest that the serotonergic system exerts facilitative and inhibitory influences on different systems in regulating reflexive erection. On the other hand, serotonin appears to play an inhibitory role in the induction of noncontact erection, because pCPA did not directly induce erection, but rather facilitated the response to females.