1. EEG and auditory event related potentials (ERPs) were recorded in monkeys following administration of water, naloxone (1, 10 mg/kg), or ethanol (0.75, 1.5 g/kg) plus naloxone (1 mg/kg). 2. Significant increases in EEG slow waves in the 2-8 Hz frequencies were seen following the 10 mg dose of naloxone. No significant EEG effects were observed at the 1 mg dose. 3. Naloxone was found to decrease the amplitude of the N1 component at the low doses and increase the N1 amplitude at the higher doses, whereas both doses reduced the latency of the P2 and P3 components of monkey auditory ERPs. 4. Low doses of ethanol (0.75 g/kg) and Naloxone (1 mg/kg) were found to produce a decrease in amplitude of the P3 response when compared to administration of naloxone alone. Whereas higher ethanol doses reversed some of the actions of naloxone on the latency of the P2 and P3 component of the ERP. 5. These studies suggest that opioid systems may participate in some aspects of attention or stimulus evaluation in the squirrel monkey.