Transforming growth factor-beta1 (TGF-beta1) 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-beta1 or neutralizing TGF-beta1 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-beta1 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-beta1 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-beta1, it induced a shift to a less slow MHC phenotype and contractile characteristics. Conversely, neutralization of TGF-beta1 in the regenerating fast muscle induced a shift to a less fast MHC expression. Together these results suggest that TGF-beta1 influences some aspects of fast muscle-type patterning during skeletal muscle regeneration.