A quantitative study of the melanin-containing dopaminergic neurons of the nigrostriatal system (A 9) and of the mesolimbic system (A 10) was carried out on Nissl-stained serial sections of nine normal brains, six age-matched brains of schizophrenics, five brains of paralysis agitans, and six brains of postencephalitic parkinsonism. By contrast with most laboratory animals the A 10 cell group is not well developed in the human brain. Both in Parkinson's disease with a known hypoactivity of dopamine neurons and in schizophrenia with a postulated hyperfunction of these systems, pathological alterations of the dopamine cell groups can be observed. In paralysis agitans the nigrostriatal and the mesolimbic cell groups exhibit a significant loss of neurons, while the remaining mesolimbic cells appear to be in better condition. In the postencephalitic parkinsonism both systems have almost completely disappeared with a significant loss of nerve and glial cells. In schizophrenia there is a significant decrease in the volume of the nigrostriatal area. Here the mean volume of the glial nuclei is reduced, whereas the mean volume of the nerve cells is diminished in the mesolimbic part.