Neuronal loss in the substantia nigra of patients with Parkinson's disease (PD) does not occur evenly throughout the nucleus: the ventrolateral part of the substantia nigra degenerates more severely, whereas the medial part is relatively preserved. This pattern of nigral neuronal loss is compatible with the uneven loss of dopamine in the striatum (the putamen more affected than the caudate nucleus). The predominant loss of ventrolateral nigrostriatal projections in PD, leading to substantial loss of dopamine especially in the putamen, is thought to contribute to the motor symptoms of the patients. On the other hand, the more medial nigral projections may be involved in the cognitive symptoms of patients. Selegiline (L-deprenyl) has been shown to delay the need to initiate levodopa therapy in early PD, and selegiline has also been suggested to increase the survival of PD patients. These observations have led to the proposal of selegiline's neuron-saving effect in PD. There is some pathological evidence supporting the better survival of nigral neurons in PD patients treated with selegiline as compared with those without such treatment. Further studies are, however, needed to elucidate this question more clearly.