The rev-erbA orphan protein belongs to the steroid nuclear receptor superfamily. No ligand has been identified for this protein and little is known of its function in development or physiology. In this study, we focus on (1) the distribution of the rev-erbA protein in adult fast- and slow-twitch skeletal muscles and muscle fibers and (2) how the rev-erbA protein influences myosin heavy chain (MyHC) isoform expression in mice heterozygous (+/-) and homozygous (-/-) for a rev-erbA protein null allele. In the fast-twitch extensor digitorum longus (EDL) muscle, the rev-erbA protein expression was linked to muscle fiber type; however, MyHC isoform expression did not differ between wild-type, +/- or -/- mice. In the slow-twitch soleus muscle, the link between rev-erbA protein and MyHC isoform expression was more complex than in the EDL. Here a significantly higher relative amount of the /slow (type I) MyHC isoform was observed in both rev-erbA -/- and +/- mice than in wild-type controls. A role for the ratio of thyroid hormone receptor proteins 1 to 2 in modulating MyHC isoform expression can be ruled out, since no differences were seen in MyHC isoform expression between TR2 deficient (heterozygous and homozygous) and wild-type mice. Therefore, our data are compatible with the rev-erbA protein playing an important role in the regulation of skeletal muscle MyHC isoform expression.