The lever-pressing of rats was reinforced with food according to a variable-interval 1-min schedule. In one group, occasional illumination of cue lights for 30-sec periods was followed by a brief electric shock; responding was suppressed during these periods. Naloxone (0.01-10 mg/kg) did not change the degree to which responding was suppressed during the pre-shock stimulus. Diprenorphine (0.1-10 mg/kg) slightly attenuated suppression, and diazepam (1.0-3.0 mg/kg) increased responding during the stimulus to normal levels. These results confirm that opiate antagonists do not always enhance the effects of shock on behavior. In a second group, occasional illumination of the cue lights for 20-sec periods was followed by delivery of free food pellets. Responding was also suppressed during the pre-food stimulus. Neither naloxone nor diprenorphine had any effect on response rate during this stimulus. In contrast to the results of earlier studies using benzodiazepines, diazepam (1.0-3.0 mg/kg) produced a marked attenuation of response suppression during the pre-food stimulus.