Rats treated with high dose of thyroid for 8 days produced a cardiac and renal hypertrophy. The cardiac hypertrophy was proportionately greater than renal hypertrophy. The specific activity of cardiac aldolase was elevated to 178 of that in the control animals after 9 days of daily hormone treatment, the biggest change seen between the 8th and 9th days of T₃ administration. The rates of synthesis and degradation as determined by the changes in specific radioactivity and total radioactivity, respectively, both fluctuated but in such a way that the net effect was small.