ABSTRACT. Muscle regeneration is a potentially useful model for defining the mechanisms responsible for nerve-dependent myosin isogene regulation in skeletal muscle. As a first step towards this goal we have characterized the pattern of expression of the four myosin heavy chain (MHC)genes, MHC-^/slow, -2A, -2X and -2B isogenes, during early stages of muscle regeneration both in the presence and in the absence of the nerve. Muscle degeneration/regeneration was induced by intramuscular injection of the myotoxic drug, bupivacaine, in the rat slow soleus muscle. MHC transcripts were identified by in situ hybridization with specific riboprobes during the period from day 3 to day 7 after muscle injury. The four genes are not detected at day 3, when the regenerating muscle contains predominantly embryonic and neonatal MHC isoforms. MHC-2X and -2B transcripts are first detected at day 4 in both innervated and denervated muscles. These transcripts remain as major transcripts in denervated muscles whereas they are down-regulated by day 5 and disappear by day 6-7 in the presence of the nerve. Innervation induces strong up-regulation of MHC-2Aat day 4 and MHC-^/slow transcripts at day 5. MHC-2A transcripts are first homogeneously expressed in most fibers then become segregated in a minor population of fibers by day 6-7 while MHC-^/slow transcripts increase in most fibers. In the absence of the nerve MHC-^/slowtranscripts are never expressed and MHC-2A transcripts are detected in rare fibers at days 5-7.