During animal development, cells become progressively more restricted in the cell types to which they can give rise. In the central nervous system (CNS), for example, multipotential stem cells produce various kinds of specified precursors that divide a limited number of times before they terminally differentiate into either neurons or glial cells. We show here that certain extracellular signals can induce oligodendrocyte precursor cells to revert to multipotential neural stem cells, which can self-renew and give rise to neurons and astrocytes, as well as to oligodendrocytes. Thus, these precursor cells have greater developmental potential than previously thought.