TREMOR, resembling that seen in parkinsonism, can be produced by placing a lesion in the ventromedial mesencephalic quadrant. The structures which need to be involved in such an approach and their relative importance have been described in a number of earlier communications.1-4The fact that such tremor takes some time to mature and tends to change its character with the passage of time limits its utility as a paradigm for research in parkinsonism. The principle of temporal change has, however, important implications for an understanding of the etiology of that common clinical disorder. The principle of significant dimension in time is not limited to tremor but can be found in other types of symptomatology, such as lid retraction, and occurs following a wide variety of types of initial "causes." A time lag after the onset of the supposed etiologic process in some patients with parkinsonism has long been known.