Ten patients with idiopathic Parkinson's disease and motor fluctuations were rated for mood changes during discrete "off," "on," and "on with dyskinesia" periods. The Profile of Mood States and visual analogue scales were used. Significant changes in mood and anxiety were found to parallel changes in motor fluctuations. One patient rated his moods as consistently improving from the "off" state to the "on" state and finally to the "on with dyskinesia" state, a finding that is consistent with concomitant central dopaminergic changes. All other patients showed moods that improved significantly from the "off" state to the "on" state but then worsened significantly in the "on with dyskinesia" state, a finding that is consistent with the fact that patients feel worse when impaired by dyskinesias. It is suggested that these results argue for multiple etiologies of depression in Parkinson's disease. The literature on dopamine and depression in Parkinson's disease is briefly reviewed and the opportunity provided by "on-off" phenomena to study the effect of dopamine on mood changes is discussed.