A clinical survey indicates a similar incidence of depression in old age and Parkinson’s disease. Divergencies, therefore, are apparent, when the biochemical basis of depression is related to the pathobiology of Parkinson’s disease (PD) and endogenous depression. However, a common biological concept for endogenous depression and depression in PD is possible by assuming that neurotransmission can operate at various levels of homeostasis. Then depression is the behavioral correlate rather of regional changes that disturb this homeostasis. Therefore, the incidence of depression in old age and PD is similar.