Evidence from a variety of sources indicates that the mesolimbic-mesocortical dopamine projections may play an important role in some types of reward or reinforcement processes in animals. There is circumstantial evidence that this is also true in humans. Since a reduced ability to experience pleasure or reward (i.e. anhedonia) is a cardinal feature of clinical depression, and since the mesolimbic and mesocortical dopamine projections have been shown to degenerate in Parkinson's disease, it is suggested that damage to these reward-related systems may contribute directly to the high incidence of depression that has been reported in this disease.