The temporal changes in intensity of myelination of the nervous pathways in 0 to 42-day-old Wistar rats were quantitatively analyzed by immunohistochemistry with anti-proteolipid protein and compared with that obtained by immunohistochemistry with anti-myelin basic protein. Immunohistochemistry was performed on paraffin-embedded tissue according to the standard ABC technique. Intensity of myelination was examined by an image analyzing system. We analyzed nine nervous pathways: corpus callosum, optic tract, internal capsule, spinal tract of the trigeminal nerve, inferior cerebellar peduncle, cerebellar white matter, pyramidal tract, medial longitudinal fasciculus, and cuneate fasciculus. The presence of immunoreactive fibers for proteolipid protein (PLP) in the spinal tract of the trigeminal nerve, medial longitudinal fasciculus and cuneate fasciculus was noted on postnatal day 0. Those of the corpus callosum, inferior cerebellar peduncle, cerebellar white matter, pyramidal tract and internal capsule were noted on day 7, and that of optic tract on day 14. The time required to reach the intensity of myelination of day 42 was day 14 for the cuneate fasciculus, day 21 for the spinal tract of the trigeminal nerve, inferior cerebellar peduncle and medial longitudinal fasciculus, day 28 for the optic and pyramidal tracts, day 35 for the corpus callosum and day 42 for the internal capsule and cerebellar white matter. The appearance of immunoreactive fibers for PLP was usually earlier than that for myelin basic protein (MBP) and the pattern of difference between PLP and MBP can be classified into three groups: (1) their time of appearance and progress are almost the same, as in the optic tract; (2) the appearance and progress of PLP occur earlier than those of MBP, as in the pyramidal tract; (3) the appearance of PLP occurs earlier than that of MBP, but their progress is the same. Our findings revealed that the time of appearance and progress of myelination as measured by PLP are different among the nervous pathways, and that there is also a difference between PLP and MBP. This difference between PLP and MBP may indicate a functional difference between them. (C) 1998 Elsevier Science B.V. All rights reserved.