Learned associations between effects of abused drugs and the drug administration environment are important in drug addiction. Histochemical and electrophysiological studies suggest that these associations are encoded in sparsely distributed nucleus accumbens neurons that are selectively activated by drugs and drug-associated cues. Although correlations have been observed between nucleus accumbens neuronal activity and responsivity to drugs and drug cues, no technique exists for selectively manipulating these activated neurons and establishing their causal role in behavioral effects of drugs and drug cues. Here we describe a new approach, which we term the 'Daun02 inactivation method', that selectively inactivates a minority of neurons previously activated by cocaine in an environment repeatedly paired with cocaine to demonstrate a causal role for these activated neurons in context-specific cocaine-induced psychomotor sensitization in rats. This method provides a new tool for studying the causal roles of selectively activated neurons in behavioral effects of drugs and drug cues and in other learned behaviors.