Four juvenile male baboons were trained on a delayed match-to-sample (MTS) discrimination task. Single administration of 0.25, 0.5, 1.0, 2.0, 4.0, and 6.0 mg/kg diazepam (DZ) resulted in a dose-related increase in mean response time (MRT), with a maximum effect at 2.0 mg/kg. Administration of 0.25 mg/kg of DZ had no effect. Response time was increased significantly by 1.0 g/kg of ethanol (EtOH) but was not affected by 0.5 g/kg EtOH. There were no significant effects on the number of correct responses. Combined treatment of 0.5 g/kg of EtOH with 1.0 mg/kg DZ produced the same increase in MRT as the DZ alone. When 1.0 g/kg EtOH was administered with 0.25 mg/kg DZ. MRT decreased which was a significant improvement in performance as compared to the increase in MRT observed with either the EtOH or the DZ alone. Combined doses of 1.0 g/kg EtOH and 0.5 mg/kg DZ produced essentially the same increases in MRT as either dose alone. Similar results were obtained when 1.0 g/kg EtOH and 1.0 mg/kg DZ were combined and the increase in MRT was about the same for DZ alone and more than twice as great for the EtOH alone. Combined administration of the drugs had no significant effects on the number of correct choices. Since the administration of a combination of a low dose of DZ with EtOH appeared to prevent the increases in MRT observed with either drug alone and there were no observable effects on the number of correct response, these effects do not appear to involve cognitive processes but might be related to excitability changes in motor systems.