• We present the thesis that many drugs of abuse are used for their hedonic effects and that a relevant animal model for the study of these effects is the action of these drugs on the pathways that support rewarding intracranial self-stimulation. A relationship between abuse potential of a drug and its ability to lower the threshold for rewarding brain stimulation in the rat was found. Of all the compounds we have studied, morphine and cocaine were the drugs that caused the maximum lowering of the rewarding threshold. Phencyclidine hydrochloride and the mixed agonist-antagonist pentazocine also lowered the threshold to a lesser degree, while the mixed agonist-antagonists cyclazocine and nalorphine hydrochloride had inconsistent effects. Naloxone hydrochloride, at the doses tested, had no effect on the threshold. Further, there is no evidence that tolerance develops to the threshold-lowering effect of morphine, suggesting that continued use of narcotics by the physically dependent individual is not simply due to an effort to avoid the pain of withdrawal.