The real time RT-PCR measurement confirmed the expression of all four MyHC mRNAs (type 1, 2a, 2x/d and 2b) in both fast extensor digitorum longus (EDL) and slow soleus (SOL) muscles, the relative proportion of fast 2a : 2x/d : 2b MyHC isoform transcripts in the SOL muscle were, however, by two to three orders of magnitude lower than that of slow MyHC 1 isoform. This result is in agreement with the dominant content of MyHC 1 protein isoform supplemented by a small amount of MyHC 2a isoform usually found in the adult rat soleus muscle