To understand the molecular basis of the functional decline in aging muscle, we examined the functional (actomyosin ATPase) and chemical (cysteine content) changes in actin and myosin purified from the muscles of young (4- to 12-month-old) and old (27- to 35-month-old) Fisher 344 rats. Using the soluble, catalytically active myosin fragment, heavy meromyosin (HMM), we determined the maximum rate (V(max)) and actin concentration at half V(max) (K(m)) of the actomyosin ATPase, using four combinations of actin and HMM from old and young rats. V(max) and K(m) were significantly lower when both actin and HMM were obtained from old rats than when both proteins were obtained from young rats. The number of reactive cysteines in HMM significantly decreased with age, but no change was detected in the number of reactive cysteines in actin. We conclude that aging results in chemical changes in myosin (probably oxidation of cysteines) that have inhibitory effects on the actin-activated myosin ATPase.