The effects of morphine, diazepam and chlorpromazine were examined on the discrimination of electric shock in squirrel monkeys. Two discrete responses measured discrimination of the presence or absence of shock at two different intensities, either at 0.35 mA or at an intensity determined to be close to each monkey's threshold (0.05 or 0.15 mA). The percentage of responses which were correct in the presence and the absence of shock were determined separately. Time to respond to the presence or the absence of shock was also determined. Morphine decreased the percentage of correct responses at both the lower (0.05--0.15 mA) and the higher (0.35 mA) shock intensities, although effects at the higher intensity were very slight. Morphine had little effect on correct responses in the absence of shock. In contrast, diazepam decreased the percentage of correct responses in the absence of shock without affecting correct responses in the presence of shock, and chlorpromazine decreased the percentage of correct responses both in the presence and absence of shock. All three drugs increased time to respond both to the presence of shock and to the absence of shock with chlorpromazine producing the largest increase. Only morphine increased time to respond in the absence of shock more than in the presence of shock.