Oligodendrocyte (OL) progenitors in the mouse spinal cord are generated from a strictly restricted region in the ventricular zone of the ventral cord as early as on embryonic day 11 (E11). We previously reported that one of the factors that restrict this ventral specific appearance of OLs is an inhibitory factor secreted from the dorsal spinal cord, in addition to well-known stimulatory ventral factors such as sonic hedgehog. We characterized the developmental change of the inhibitory activities. They were very strong at E11, gradually reduced, and disappeared by E14. This pattern seemed to be well correlated with the developmental profile of Wnt3a expression at/near the roof plate. A conditioned medium of L cells that stably express Wnt3a showed significant reduction of O4 positive OLs in the ventral spinal cord explants, indicating that Wnt3a is one of the dorsal factors that inhibit OL development. Addition of Wnt3a supernatant to CG4 cells, an OL progenitor strain, and to the dissociated primary cultured cells suggested that Wnt3a directly acts on OL lineage cells and inhibits a differentiation step from OL progenitor to O4-positive stage. Thus, Wnt3a may directly control the timing of OL differentiation and the motility of OL lineage cells. A population of myelinating OLS in the dorsal area of telencephalon was further demonstrated to be ventral origin by the newly established cell marking system using in utero DNA electroporation.