DNA topoisomerases are enzymes that catalyze topological changes of DNA by transiently cleaving one (topo Ⅰ) and a pair of complementary DNA strands (topo Ⅱβ), participating in cellular transactions of DNA. Expression of both enzymes in the developing cerebellar plate in rat embryos was studied immunohistochemically. Neuroepitheliad cells in the ventricular layer expressed topo Ⅰ and topo Ⅱβ on embryonic day 15 (E16) and E16, respectively. On E 16-22, cells in the intermediate layer (IML) are known to undergo extensivc differentiation to become Purkinje cells and deep cerebellar neurons. Many IML cells, immunostained with not only anti-topo Ⅰ and-Ⅱβ but also-calbindin antibodies, were differentiating and migrationg to form the Purkinje cell layer. Cells in the external germinal layer (EGL) on E17, originating from the germinal trigone and spreading over the surface of cerebellar plate, expressed topo Ⅰon E17. Topo Ⅱβ immunoreactive cells, however, were not readily recognized in EGL untill E22. The topo Ⅱβ immunopositive cells resided in the ventral half of the layer, corresponding to the premigratory zone of postnatal EGL. These date suggest the involvement of topo Ⅰand Ⅱβ in DNA metabolism associated with cerebellar development in embryos, especially that of topo Ⅱβ in the processes of neuronal differentiation.