A convergence of clinical and pathological evidence points to the basal ganglia as the site of disturbance in compulsive disorders. However, the limbic system may be implicated as well. This article draws upon various lines of animal research in an attempt to explain how disturbances in one or another of these systems may produce compulsive behavior. Possible models include stimulation of the reinforcement mechanism, manipulation of the striatal "comparator" function, production and blockade of displacement behavior, and interference with the hippocampus' modulation of the stereotypy-inducing effect of reward. The common denominator of these models is a relative excess of dopaminergic activity in the basal ganglia. However, this does not necessarily implicate a primary dopaminergic disturbance in all human compulsive behavior.