Immunocytochemistry and thymidine autoradiography were combined to determine the time elapsed between cell division and the expression of proteolipid protein (PLP) in individual oligodendrocytes in normal mouse brain. In jimpy (jp) brains, autoradiography was used to determine the time elapsed between cell division in an individual oligodendrocyte and evidence of cell death. Oligodendrocytes in normal mouse brain do not express PLP until 72 h after a single injection of [³H]-thymidine. In contrast, oligodendrocytes in jp brains begin to die within 9–11 h after an injection of thymidine. The jp mouse is one of several X-linked, hypomyelinated mutants in which a defect has been demonstrated in the gene coding for PLP. It has been presumed that the lack of this protein in the myelin sheath is responsible for the jp phenotype. However, the present study shows that individual jp oligodendrocytes begin to die long before they would normally have synthesized detectable levels of PLP. Therefore, it seems unlikely that the death of jp oligodendrocytes is due to the absence of PLP in myelin sheaths. Oligodendrocyte death and other early jp abnormalities may be due to the presence of abnormal PLP message which may interfere with glial differentiation. Alternatively, the PLP message may code for another protein which is important for normal development of neuroglia.