19 patients with bipolar affective disorders were investigated with visual averaged evoked responses (V.AER) to different stimulus intensities. 7 patients were investigated only in the depressive phase, 8 only in the manic phase, 2 in both the depressive and the manic phase, 1 in the manic phase and after recovery, and 1 patient was followed in both the depressive and the manic phase and when recovered. Both the comparisons between the independent groups and the results from the individual patients followed in different phases of the disorder indicate that patients in the depressive phase have a tendency to decrease in maximum amplitude when stimulus intensity is increased (reducing). Patients in the manic phase, on the other hand, show increasing maximum amplitude with increasing stimulus intensity (augmenting). The biochemical basis for this switch in augmenting/reducing tendency seem to be changes in the activity in the monoaminergic systems.