Induction of cardiac hypertrophy by thyroxine was compared in adult (8 mo.) and old (26 mo.) euthyroid C57BI/6J mice. Hearts in old and adult animals ultimately exhibited the same percentage increases in heart/body weight ratio, and dose-response curves were identical, but there was a significantly longer lag period before hypertrophy occurred in older mice. in both groups, increases in 3H-leucine incorporation into protein were observed prior to increases in heart/body weight ratios. We interpret these observations to indicate that lower protein synthetic rates in old hearts signal an impairment in adaptability which may contribute to the increased incidence of heart failure with age.