The performance of normal human volunteers and marmosets on a 2-choice guessing task was assessed after saline (control) or amphetamine administration. In human subjects the drug increased the number of alternation responses, which can be interpreted as an increase in stereotyped switching and which is similar to the response pattern produced by some groups of psychotic patients on the same task (Frith and Done 1983; Lyon et al. 1986). Marmosets treated with amphetamine showed an increase in perseverative responding compatible with that seen on other types of task. Our conclusion is that dopaminergic systems are involved in behavioural choice mechanisms and that a dysfunction of these systems may contribute to the symptomatology of psychosis.