We have prepared a XgtlO cDNA library with the mRNA isolated from fetal calf brains which were actively myelinating. Using two oligonucleotides made according to the known amino acid sequence of myelin proteolipid protein (PLP or lipophilin), we have isolated several cDNA clones for this major intrinsic membrane protein of myelin. One of these clones, designated as pLPl, is found to contain 444 bp of coding sequence for the C-terminal half of PLP and 486 bp of 3' untranslated sequence. Using pLPl as a hybridization probe, we have studied the regulation of PLP at the mRNA level during rat brain development. Our results show that the relative amounts of mRNA for PLP and that for the major extrinsic protein of the myelin membrane, myelin basic protein, increase coordinately during the course of myelination in the brain.