The induction of penile erections by a variety of compounds with a direct or indirect effect on serotonin (5HT) receptors was investigated in rats. L-5-Hydroxy-tryptophan (L-5HTP) induced penile erections when co-administered with nialamide and the peripheral decarboxylase inhibitor benserazide, indicating that the site of action for inducing penile erections is within the central nervous system. Penile erections were also induced by the 5HT uptake inhibitors zimelidine, fluoxetine, citalopram, Org 6997, by the 5HT-releasing agent fenfluramine and by the putative 5-HT1B receptor agonist 1-(3'-chlorophenyl)-piperazine (mCPP). The 5HT1A-agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) did not induce penile erections. The less selective 5HT receptor agonists 5-methoxy-N,N-dimethyl-tryptamine (5MeODMT), 5-methoxytryptamine (5MeOT), dl-lysergic acid diethylamide (LSD) and Ru 24969 were also ineffective. Induction of penile erections by quipazine appeared only when this compound was co-administered with the 5HT2 receptor antagonist pirenperone. Receptor antagonists were tested against penile erections induced by Org 6997. The beta-adrenoceptor antagonists that also have 5HT1 antagonistic properties, (S)-pindolol and dl-propranolol, antagonized Org 6997-induced penile erections but butoxamine and metoprolol did not. Spiperone and pirenperone in doses selective for 5HT1A and 5HT2 receptors respectively were also inactive. Haloperidol, 0.46 mg/kg, partially attenuated penile erections induction. The data are discussed in the light of the hypothesis that penile erections induction by serotonin-mimetic compounds is mediated by 5HT1B receptors in the striatum.