After extensive training on a variable cycle 15-s schedule of shock avoidance, rats were exposed to shock intensity variations from 0.1 to 0.8 mA. In addition, prior to each session rats received injections of a moderate dose of the opiate antagonist, naloxone (3 mg/kg), or saline vehicle. Response rates and percentage of shocks avoided increased with increasing shock intensity for all three rats. At the high shock intensities naloxone depressed response rates and percentage of avoidance. This naloxone-induced impairment of avoidance was particularly evident in the latter part of the 1-hr sessions. However, at the lower shock intensities, naloxone had no effect for two animals and appeared to increase performance in the third rat. Effects of naloxone on variable-cycle avoidance thus depended on the programmed shock intensity.