ABSTRACT Male Wistar rats were treated with 1-thyroxine, 1-triiodothyronine, triiodothyroacetic acid and triiodothyropropionic acid in equimolar doses. The catabolic effect (body and organ weight), the variation of the heart weight, the augmentation of tissue respiration, the DNA-content of liver and the activity of enzymes in red blood cells have been compared.