The opioid LY150720 is a racemic mixture whose resolution results in a highly stereospecific separation of agonist and antagonist activity. The effects of LY150720 (0.3-3.0 mg/kg), its agonist (dextro) isomer LY136596 (0.3-1.7 mg/kg) and morphine (0.03-1.0 mg/kg) were studied alone and in combination with naloxone (0.001-1.0 mg/kg) in squirrel monkeys whose responding was maintained under a multiple fixed-ratio 30-response fixed-interval 5-minute (mult FR-30 FI 5-min) schedule of food presentation. LY150720, LY136596 and morphine generally decreased responding under both schedule components, although in several instances increases in responding under the FI component were noted, particularly following LY150720 and LY136596. Naloxone (0.1-3.0 mg/kg) generally had little effect on responding, whereas the antagonist (levo) isomer LY136595 (0.3-10.0 mg/kg) decreased responding under both schedule components. The rate-decreasing effects of morphine, LY150720 and LY136596 were reversed by naloxone; doses of naloxone required to reverse the effects of all three drugs were comparable. When combined with morphine, naloxone restored rates and patterns of responding to control values, whereas combinations of LY150720 or LY136596 and naloxone increased responding under the FI component in excess of control values. These increases appear to be due to anticholinergic actions of LY150720 and LY136596, as they are reversed by physostigmine (0.01 mg/kg) and similar increases are produced by scopolamine (0.01-0.1 mg/kg).