Abstract
O-Methyl-DL-thyroxine was approximately twice as active as DL-thyroxine in causing tail shrinkage in R. clamitans tadpoles. It was 1/3 as effective as DL-thyroxine in stimulating O2 consumption of rats but was only slightly active in preventing thyroid enlargement of thiouracil-fed rats. Thiouracil feeding did not prevent the effect of O-methyl thyroxine on basal metabolic rate of rats.