Several proerectile drugs act on the nitric oxide–cyclic guanosine monophopsphate pathway, which is known to influence rat copulatory behavior. In the present study we evaluated the effects of two proerectile compounds, one (Impaza) acting on endothelial nitric oxide synthase, and the other (sildenafil) on phosphodiesterase 5, on sexual incentive motivation in male rats displaying a spontaneously low level of motivation and copulatory behavior. About 20 months old male Fisher 344 rats were tested in a procedure for evaluating the intensity of sexual incentive motivation and in standard mating tests. For comparison, a group of young (about 4 months) Fisher 344 males was tested in parallel. This group did not receive any drug treatment. Impaza was administered in two doses, daily for 28 days, and sildenafil was given at a dose of 3 mg/kg twice a week during 28 days. Tests for sexual incentive motivation and copulatory behavior were performed immediately before the beginning of drug treatments, and on days 7, 14 and 28 of treatment. All treatment groups displayed a very low level of copulatory behavior and a sexually receptive female was not a more powerful incentive than another male at the tests performed before and on days 7 and 14 of treatment. On day 28 of treatment, the group treated with Impaza, 3 ml, displayed a preference for the sexually receptive female, while no such preference was found in the other groups. Furthermore, the preference score was above that of controls in this group. Both Impaza, 3 ml, and sildenafil reduced approach to the male in the test for sexual incentive motivation, suggesting that social motivation was reduced. These data suggest that compounds affecting the nitric oxide–cyclic guanosine monophopsphate pathway may modify both sexual and social motivation in old rats.