The notion that motivated behaviors having an energetic or activational component is an old one, and there are numerous examples of this idea from the literatures of psychology and ethology. Behavioral researchers have demonstrated that vigor or persistence of work output in stimulus-seeking behavior is a fundamental aspect of motivation. In addition, psychiatrists and clinical psychologists have come to emphasize the importance of energy-related dysfunctions, such as psychomotor slowing and apathy, in various clinical syndromes. Because of the potential scientific importance and clinical relevance of behavioral activation processes, it is critical to determine the brain mechanisms that are involved. Considerable evidence indicates that DA in nucleus accumbens is involved in activational aspects of motivation. Accumbens DA depletions decrease spontaneous, stimulant-induced, and food-induced motor activity. In addition, the effects of accumbens DA depletions on food-seeking behavior depend greatly upon the response requirements of the task. Research involving concurrent choice tasks has shown that rats with accumbens DA depletions reallocate their instrumental behavior away from food-reinforced tasks that have high response requirements, and instead select a less-effortful type of food-seeking behavior. Rats with accumbens DA depletions are particularly sensitive to lever pressing schedules with high ratio requirements (i.e., a large number of lever presses must be emitted for reinforcement to occur). Together with studies of frontal cortex function in animals, and clinical studies on neurochemical and other functional changes in the human brain, this line of research could have implications for understanding the brain circuitry involved in energy-related psychiatric disorders such as psychomotor slowing in depression, anergia, fatigue and apathy.