Using postmortem immunohistochemical analysis, we have identified degeneration of several different neuronal cell groups in the brainstem of patients dying with idiopathic Parkinson's disease. We report the first chemically identified loss of presumed serotonin neurons in the median raphe nucleus of the pons and of substance P-containing preganglionic neurons in the dorsal motor vagal nucleus. This evidence is concordant with other evidence that the primary neuropathological process is not confined either to a single pathway or to neurons containing a particular transmitter. Rather it appears that Parkinson's disease affects several classes of neurons in localized areas of the brainstem.