Pigeons were monocularly trained on FR 40. Then, unilateral spreading depression was produced by a micro-injection of Kcl solution into the hyperstriatum. Spreading depression on the trained hemisphere (contralateral to the trained eye) completely suppressed the responding of the subjects, but spreading depression on the untrained hemisphere (ipsilateral to the trained eye) did not cause complete suppression. However, the depression on the untrained hemisphere had a transient decreasing effect on the responding of the subjects. The present results demonstrate the lateralization of monocular FR training in that the intact trained hemisphere is necessary to maintain a high rate responding. EEG was continuously monitored during the experiment, but the duration of EEG depression, did not agree well with the duration of behavioral suppression.