Previous work has found that N-acetylcysteine inhibits extinction responding in rats trained to self-administer heroin. The current study examined the ability of N-acetylcysteine to inhibit extinction responding in rats trained to selfadminister cocaine. Rats were trained to self-administer cocaine (0.39mg/kg) for 10 to 12 days and were pretreated with either N-acetylcysteine (60mg/kg) or saline beginning on the first day of extinction training and on each extinction training day thereafter. Results indicated that chronically administered N-acetylcysteine reduced lever pressing during extinction sessions. In addition to demonstrating the impact N-acetylcysteine has on lever pressing during extinction, the present study underscores the importance of using responding during extinction as a dependent measure in the development of medications for addictive behaviors.