Muscleregeneration is a potentially useful modelfor defining the mechanismsresponsible for nerve-dependent myosinisogene regulation in skeletal muscle. Asa first step towards this goal wehave 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. MHCtranscripts 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, whenthe regenerating muscle contains predominantly embryonic and neonatal MHCisoforms. MHC-2Xand-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-^/slowtranscripts at day 5. MHC-2Atranscripts are first homogeneouslyexpressed in most fibers then becomesegregated in a minorpopulation of fibers by day 6-7 while MHC-^/slowtranscripts increase in most fibers. In the absence of the nerve MHC-^/slowtranscripts are never expressed and MHC-2Atranscripts are detected in rare fibers at days 5-7.