Under baseline conditions, responding was maintained by intravenous cocaine delivery (100 micrograms/kg/infusion) under a fixed ratio 10 schedule in three rhesus monkeys. During test sessions, the onset of each cocaine infusion was accompanied by a delivery of electric shock of pre-determined duration and intensity. At intermediate intensity levels, the electric shock delivery initially reduced cocaine maintained responding. Although test sessions were separated by at least three baseline sessions, adaptation to the punishing effect of shock occurred within five test sessions in each monkey. Adaptation did not occur at higher intensity levels which completely eliminated cocaine-maintained responding, even when this intensity was tested prior to intermediate intensity levels.