A series of experiments was performed to determine the effects of naloxone on the acquisition of conditioned suppression in rats. Conditioned suppression acquisition functions are often nonmonotonic due to within-session performance decrements and to postasymptotic performance decrements. Naloxone given during acquisition increased post-US suppression (Experiment 1 ) , depressed baseline response rates in a situation involving shock presentation (Experiments 1 and 2b) t eliminated the postasymptotic performance decrement (Experiment 1), and caused greater resistance to extinction than did saline (Experiments 1 and 2b). Naloxone did not eliminate within-session decrements (Experiment 1 and 2b) and failed to affect extinction of conditioned suppression when administered only during extinction (Experiments 1 and 3). Naloxone sliightly decreased baseline responding for sucrose reinforcement. (Experiment 2a), but this effect was a small one and did not confound the effects of naloxone on conditioned suppression. The results suggest that naloxone affects the acquisition of conditioned suppression by increasing the aversiveness of the shock US. The possible role of the activation of an endorphinergic system during fear conditioning is discussed.