Six oculorotatory muscles and the levator palpebrae muscle of the rat were analysed by SDS-PAGE for their myosin heavy chain (MHC) isoform patterns. Oculorotatory muscles display a marked predominance of fast MHC isoforms. They contain, in addition to the slow (MHCI) and fast (MHCIIb, MHCIId, MHCIIa) skeletal MHCs, the neonatal MHCneo and the extraocular MHCeom. The levator palpebrae, generally assumed to be a member of the extraocular muscle group because of its innervation by the oculomotor nerve, does not contain MHCneo and MHCeom. It resembles a fast-twitch skeletal muscle with a predominance of MHCIId.