Homotopic and isochronic transplantations of the right dorsal half of the mesencephalic vesicle have been performed between chick and quail embryos at the stage of 10 - 14 somites. Analysis of the extension of the graft, by means of the quail nucleolar marker, combined with cytoarchitectonic analysis has disclosed that the transplanted neuroepithelium gives rise to isthmic nuclei and to a portion of rostral cerebellum, in addition to the optic tectum and mesencephalic dorsal grisea. These results show that the rostral portion of the cerebellar primordium is located in the so-called 'mesencephalic' alar plate, thus considerably more rostrally than previously supposed. This has been confirmed by two other types of chimeric embryos resulting from homotopic transplantation of either: (i) the quail right alar plate of the first rhombencephalic vesicle, which gives rise to caudal but not rostral cerebellum in the operated side, or (ii) the right alar portion of a segment of the quail neural tube including both the caudal third of the mesencephalic vesicle and the rostral half of the first rhombencephalic vesicle, which gives rise to the whole hemicerebellum in the operated side. Moreover, in chimeric embryos with transplants restricted to the mesencephalic alar plate, the grafted portion of the cerebellar primordium gives rise both to deep cerebellar neurons and to all types of cortical neurons. Among the quail cortical neurons, the Purkinje cells, although intermingled with host Purkinje cells, are organized, at E18, in a characteristic longitudinal band which is strongly reminiscent of the longitudinal functional and morphological organization of the cerebellum. Other types of quail cortical neurons, that is, Golgi cells, granule cells, and molecular layer interneurons, are also observed within this sagittal band. In addition, quail granule cells and molecular layer interneurons as well as quail glial cells, extend over a larger territory on both sides of the longitudinal band containing quail Purkinje cells and even cross the midline and invade the contralateral hemicerebellum. In all types of chimeric embryos, the proliferation, migration, and differentiation of quail transplanted neurons, both in the isthmic region and in the cerebellum, evolve asynchronously from the host homologous ones, following a more precocious and faster developmental schedule. This asynchrony in the development of grafted and host isthmic and cerebellar homologous areas confirms and extends previous findings concerning the proliferation and migration of quail tectal cells in chick quail chimeric embryos (Senut and Alvarado-Mallart, 1987).