Male rats show four to six penile erection episodes when put for 80 min in the presence of an inaccessible receptive female. These non-contact penile erections were reduced dose-dependently by d(CH2)5Tyr(Me)2-Orn8-vasotocin, a potent and selective oxytocin receptor antagonist, when given into the lateral ventricles (0.1, 0.5 and 1 microg), but not when given into the paraventricular nucleus of the hypothalamus (0.1 and 1 microg). In contrast, non-contact erections were reduced by N(G)-nitro-L-arginine methyl ester, a competitive inhibitor of nitric oxide synthase, given into the lateral ventricles (50, 100 and 200 microg), or into the paraventricular nucleus (10 and 20 microg). The present results show that central oxytocin is involved in the expression of penile erection induced not only by drugs but also by sexual physiological stimuli.