The cervical spinal cords of 23-day-old myelin-deficient (md) rats, an X chromosome-linked myelin mutant, and their normal littermates were studied by light and electron microscopy, immunocytochemistry, and in situ hybridization. Light microscopy showed that there were scattered myelinated fibers in the md rat, particularly in the lateral and ventral columns. Ultrastructural examination of these fibers showed that the myelin often had many lamellae that were tightly compacted, but in which the intraperiod line was abnormally fused at most places, resulting in a minor alteration of the myelin periodicity. Immunocytochemical staining of adjacent sections following a variety of fixation methods showed that the myelinated fibers were positive for myelin basic protein but negative for proteolipid protein (PLP). In situ hybridization using cDNA probes to these proteins showed a severe diminution of the mRNAs for both proteins. These findings provide further support for an abnormality in genetic regulation of PLP as has been described in another X chromosome-linked mutant, the jimpy mouse. Despite the lack of PLP, however, a few myelinated fibers are formed in the md rat, but the myelin formed in general lacks a normal intraperiod line, a site at which this protein is thought to be located.