The effects of repeated administration of naloxone on heroin and cocaine self-administration in non-dependent rhesus monkeys were investigated. Animals lever pressing for intravenous heroin (6 micron/kg) and cocaine (100 or 200 micron/kg) were treated for 7--10 days with naloxone at a fixed dose prior to each session. Low pretreatment doses of naloxone increased rate of responding maintained by herion. The pattern of responding over the 10-day period of treatment with the higher doses of naloxone was similar to that observed when saline was substituted for heroin. Naloxone was without effect on responding maintained by injections of cocaine.