Transforming growth factor-β1 (TGF-β1) is known to be expressed in the environment of developing fast muscle fibres during ontogenesis. In the present study, we have examined effects of administration of either TGF-β1 or neutralizing TGF-β1 antibody on the induction of fast type phenotype in regenerating skeletal muscles in rats. Expressions of fast and slow myosin heavy chain (MHC) isoforms were studied using protein electrophoresis, at 3 and 6 weeks after myotoxic treatment. Muscle contractile properties were also measured in situ. The results have shown that a single injection of TGF-β1 into the regenerating slow soleus muscle increased the expression of fast MHC-2x/d and MHC-2a and decreases that of slow MHC-1 (P<0.05). Moreover, it reduced the degree of tetanic fusion during contraction (P<0.05). Conversely, injection of neutralizing antibody against TGF-β1 into the regenerating fast EDL muscle increased the expression of MHC-2a and MHC-1 (P<0.05). In conclusion, when the slow muscle was regenerating in the presence of an increased level of TGF-β1, it induced a shift to a less slow MHC phenotype and contractile characteristics. Conversely, neutralization of TGF-β1 in the regenerating fast muscle induced a shift to a less fast MHC expression. Together these results suggest that TGF-β1 influences some aspects of fast muscle-type patterning during skeletal muscle regeneration.