Summary: One specific prediction derived from the learning hypothesis is that acute or repeated exposure to addictive drugs like amphetamine can influence Pavlovian conditioned behavior and the underlying neural mechanisms. To test this prediction, three experiments were conducted in the context of a Pavlovian-conditioned approach task. In the first experiment, after animals were trained to acquire Pavlovian conditioned approach behavior, acute amphetamine was administered to evaluate the drug effect on the performance of the conditioned behavior. It was found that acute AMPH induced a dose-dependent, selective increase in Pavlovian conditioned approach behavior controlled by a reward-predictive conditioned stimulus. In the second experiment, extracellular electrophysiological recording was conducted when animals were performing the Pavlovian-conditioned approach task in a drug-free state. Neurons in the nucleus accumbens exhibited time-locked responses to the conditioned stimulus and the reward. The behaviorally relevant neuronal activity implies a neurophysiological mechanism that underlies the normal expression of the conditioned behavior. Finally, in the last experiment, the effects of amphetamine exposure on both the conditioned behavior and on the accumbal neuronal activity were measured at the same time. Amphetamine exposure enhanced both behavioral and accumbal responses to the reward-predictive conditioned stimulus, suggesting that the drug effect on the conditioned behavior was potentially mediated by the drug effect on behaviorally relevant neuronal activity. Taken together, these findings provide novel evidence that exposure to addictive drugs can modulate reward-related conditioned behavior and the underlying neural mechanisms, supporting the learning hypothesis of addiction.