A cDNA library was constructed from mRNA isolated from the brains of 18-day-old rats, the age at which myelin biosynthesis is maximal. A synthetic DNA probe synthesized based on reverse translation of the amino acid sequence of rat myelin basic protein (MBP) was used to select two cDNA clones encoding MBP. A 1.5 kb Eco RI fragment from one clone was completely sequenced. When translated, a portion of this sequence was identical at 126 of 127 positions with the reported amino acid sequence for small MBP from the rat. Brains from mice of the homozygous shiverer genotype contained neatly reduced amounts of MBP mRNA relative to wild type. A deletion of MBP sequences in the genome of shiverer mice was also demonstrated. cDNAs for MBP will allow molecular investigation of the role this gene plays in both dysmyelinating and demyelinating diseases, as well as questions of MBP biosynthesis.