Marmosets were trained on a task involving simultaneous and successive visual discrimination performance where responses were required on all trials. Performance of this task was not affected by low doses of amphetamine. From this it is concluded that amphetamine does not cause a narrowing of attention and that the disruptive effect of amphetamine on the "go-no go" successive discrimination task already reported is due to a loss of response inhibition rather than to difficulties in the recognition of stimuli presented without a comparison stimulus.