Stimulants and opiates increase synaptic dopamine (DA) transmission in nucleus accumbens (NAcc), and this action is thought to underlie the habit-forming properties of these and several other abused drugs, Much of the experimental support for this idea comes from drug self-administration studies. The fact that animals will learn an arbitrary response when it is followed by an intravenous cocaine or heroin injection has been taken to suggest that these and other such drugs act as potent rewards. It is widely assumed that the resulting increase in NAcc levels of DA is what reinforces operant-responding in animals and drug-seeking in humans. Recent evidence from a variety of sources, however, including our group, appears to challenge the validity of this assumption. In this article we review some of the findings that have emerged thus far from our in vivo electrochemical recording studies. The conclusions suggested by our research are discussed in relation to those derived from other lines of evidence.