The effects of ethanol on a conditional object identification task were investigated using an operant analog of Signal Detection Analysis. Water and three doses of ethanol (0.40, 0.75 and 1.5 g/kg) were orally administered on three separate occasions to three adult squirrel monkeys. Significant discrimination impairment as a function of increasing ethanol dose was observed. At the 1.5 g/kg dose, impairment extended to nonspecific effects, with subjects ceasing to respond early into the session. Subsequent signal detection analyses revealed that the reduction in performance resulted from losses in discriminability. Response bias was found to change unpredictably and independently of ethanol administration. Reaction time measures also showed no changes except a moderate, nonsignificant, facilitation in speed at the lowest (0.40 g/kg) dose. Taken together, these data suggest that ethanol acts to impair complex, or cognitive, performance by disrupting current sources of stimulus control within the range of doses tested.