The pathological basis for the symptoms of Parkinsonism has been the subject of much study during the last 35 years. Among the monographs devoted to it the most important are those of Tretiakoff (1919), Lewy (1923), Foix and Nicolesco (1925), Hallervorden (1935), Hassler (1938), and Klaue (1940). Four main questions are debated by these and other authors:(1) Whether the symptoms of Parkinsonism are chiefly due to lesions in the corpus striatum and globus pallidus or to lesions in the pigmented cells of the brain-stem, especially the substantia nigra.(2) Whether the lesions seen in cases of idiopathic paralysis agitans are merely the common lesions of senility with a special incidence on certain groups or typesofnerve cells.(3) Whether the lesions of idiopathic paralysis agitans are specific for the diseaseor in any way characteristic of it.(4) Whether the differences between the lesions found in idiopathic, presenile Parkinsonism (paralysis agitans) on the one hand, and those seen in cases with an earlier onset and especially those following an attack of encephalitislethargica, are qualitative or merely quantitative. Our investigation has been chiefly directed to finding an answer to the last three of these questions.