Various doses of apomorphine, deprenyl, and yohimbine were administered to old (20-26 years) rhesus males that had been sexually active when younger and to younger (6-17 years) males that were characteristically sexually sluggish. These neuropharmacological agents have been reported to increase sexual behavior in male rats. In Experiment 1, 10 old intact rhesus males were tested after injection of vehicle and apomorphine, and 6 old testosterone-treated castrated males were tested after treatment with deprenyl and yohimbine and the vehicles for each drug. In experiment 2, the 5 younger males were tested after treatment with each of the drugs and with the vehicles for each drug. There were a few minor changes in behavior associated with certain doses of each of the drugs and as many depressive as facilitative effects on sexual behavior. This suggests that there are basic differences between rats and rhesus macaques in the systems mediating sexual behavior.