Dead cells are observed in many developing animal tissues, but the causes of these normal cell deaths are mostly unknown. We show that about 50% of oligodendrocytes normally die in the developing rat optic nerve, apparently as a result of a competition for limiting amounts of survival signals. Both platelet-derived growth factor and insulin-like growth factors are survival factors for newly formed oligodendrocytes and their precursors in culture. Increasing platelet-derived growth factor in the developing optic nerve decreases normal oligodendrocyte death by up to 90% and doubles the number of oligodendrocytes in 4 days. These results suggest that a requirement for survival signals is more general than previously thought and that some normal cell deaths in nonneural tissues may also reflect competition for survival factors.