The mouse mutant Shiverer has been shown previously to lack myelin basic protein and other myelin proteins in both the peripheral and central nervous systems. Examination by electron microscopy shows that the peripheral nervous system, in contrast to the markedly abnormal central nervous system, is grossly normal. Myelin sheaths are of the usual thickness and exhibit normal periodic structure consisting of alternating major dense and intermediate lines. Subtle abnormalities do occur, however, consisting of increased numbers of cytoplasm-containing lamellae, aberrant terminations of myelin lamellae in internodal regions, invagination of the axon by the inner tongue of the myelin sheath, myelin debrin in both axon and Schwann cells, and disruption of outer myelin lamellae. Such changes have been seen previously in various types of neuropathy and are not pathognomonic of the Shiverer mutation. Despite the absence of myelin basic protein, the peripheral manifestations of this gene are relatively minor and probably not severe enough to compromise peripheral nerve function significantly.