The chemical differentiation featured by the appearance of tyrosine hydroxylase (TH) and the distribution pattern of the dopaminergic cells of rat substantia nigra (SN) were studied with combined immunocytochemical and electronmicroscopic techniques. Under the light microscope, the earliest TH-positive cells at embryonic day 13 are localized at the ventral part of rostral midbrain. Later appearing TH-positive cells join the earlier ones dorsally and caudally. As to the stain intensity and morphology of the labeled cells in the region of the SN, there exists a ventral to dorsal and lateral to medial spatiotemporal gradient, namely the cells in the ventral and lateral parts, compared with the dorsal and medial ones, have more intense staining, larger cell bodies with smaller nuclei and more and longer processes. The earliest nigrostriatal projection fibers stem from the most laterally located SN cells. Under electron microscope, rough endoplasmic reticula are always seen within the positively stained cells. With the progression of development, the cells show more intense staining and contain more rough endoplasmic reticula and other organelles. Together with the results reported on the neurogenesis and migration of the SN cells, the present study indicates that the chemical differentiation of SN cells, with a spatiotemporal gradient, starts after the completion of cell migration, a process paralleling to their morphological differentiation.