In rodents of an advanced age the cardiac contraction exhibits a reduced shortening velocity and prolonged time course. This is, in part, due to a reduction in myosin ATPase activity which results from a shift in the myosin heavy chains from the alpha (V1) to the beta (V3) isoform. The present study demonstrates that during adult aging the steady state levels of alpha myosin heavy chain mRNA decrease while those of beta myosin heavy chain increase; for both mRNAs the respective changes are greater than fourfold. Thus, the phenotypic, biophysical and biochemical cardiac contractile changes with adult aging are, at least, in part regulated at the level of gene expression.