To determine whether the relative decline in cardiac myosin isoenzyme V, with maturation continues progressively into senescence and whether thyroxine could reverse age-associated changes in the myosin isoenzyme profile and contraction, rats 2, 8, and 24 months old were treated with thyroxine, 6.4 mg/kg, for 7 days. Myosin isoenzymes, Ca2+-myosin ATPase activities, and isometric contractile function were measured in cardiac preparations from thyroxine-treated animals and age-matched controls. Right ventricular hypertrophy did not occur with aging in controls. Thyroxine increased right ventricular weight in each age group compared to the control group. Body weight decreased by 10% in all thyroxine-treated rats. The relative right ventricular V, isoenzyme content progressively decreased from 75 ± 1% to 54 ± 1% and 14 ± 1% in controls at 2,8, and 24 months, respectively, and was associated with a reciprocal increase in V3myosin isoenzyme. Ca2+-myosin ATPase activity also progressively declined monotonically with age in the control rats from 854 ± 28 nmol Pi/mg prot/min at 2 months to 529 ± 28 nmol Pi/mg prot/min at 24 months. Thyroxine administration increased right ventricular V, at each age to 97 ±2%, 73 ± 2%, and 59 ± 2% at 2, 8, and 24 months, respectively. A thyroxine induced increase in the Ca1+-myosin ATPase activity could be detected only in the 24-month-old animals. Isometric contraction duration in thin right ventricular papillary muscles increased with age from 172 ± 4 to 180 ± 5 to 225 ± 8 ms at 2, 8, and 24 months, respectively, and was markedly shortened (p<0.001) by thyroxine at each age so that the 24-month thyroxine value was less than the 2-month control. The maximum rate of force production did not change with age in controls and was increased by thyroxine at all ages. Thus, with aging from maturation to senescence, even in the absence of right ventricular hypertrophy, there is a profound monotonic decrease in percent right ventricular V, that is paralleled by a decline in Ca2+-myosin ATPase activity. Also, neither the change in genetic expression of myosin protein synthesis nor prolonged contraction duration that occurs with aging is irreversible. Both can be modulated by thyroxine.