We have isolated and sequenced a cDNA clone, homologous to the rat c-mos gene, from a cDNA library of rat skeletal muscles. The 3220 nucleotide cDNA clone codes for a protein of 339 amino acids (37.4 kDa). Both the nucleotide sequence and the deduced amino acid sequence show 60-90% overall homology to Xenopus, chicken, mouse and human mos. By Northern blot analysis, we detected two c-mos transcripts, one major of about 3.6 Kb long, and one minor of about 1.7 Kb long. These are differently regulated during the development of cardiac and skeletal muscles. By Western blot with two antibodies directed against two different portions of the mos protein, we observed in rat muscle two polypeptides of 43 kDa, and 75 kDa respectively.