The effects of oral administration of two doses of ethanol (0.75 and 1.5 g/kg) and a single dose of diazepam (2.5 mg/kg) on electrophysiological responses to an event related potential (ERP) paradigm were investigated in the squirrel monkey Saimiri Sciureus. A passive auditory "oddball plus novel" tone sequence was utilized to elicit N1, P2, N2, and P3-like components which resembled waveforms obtained from human subjects under the same conditions. Ethanol at both doses was found to produce a significant decrease in the amplitude of the N1 component. The higher ethanol dose also produced a significant decrease in the amplitude of the P3-like component. Ethanol at both doses was observed to produce significant increases in the variability of the amplitude of these components as measured from trial to trial. Diazepam, like ethanol, also induced a significant decrease in the N1 component of the ERP but did not affect the later waves. These results suggest that ERPs can reliably assess the electrophysiological actions of ethanol in monkeys.