Radiothyroidectomy delays growth of cockerels and induces a minimal hypercholesterolaemia in animal fed a normal diet but causes a marked hypercholesterolaemia in animals on a high cholesterol diet. 3:5:3′-triiodo-L-thyronine and 3:5:3′-triiodothyroacetic acid (TA3) inhibited for several months the hypercholesterolaemia of radiothyroidectomized chickens. These products decreased the levels of aortic and hepatic cholesterol. Vitamine A1 produced an elevated blood cholesterol in thyroidectomized cockerels fed with a cholesterol rich diet but given with TA3 vitamin A1 exaggerated the hypocholesteric effects of TA3 and decreased the hepatic cholesterol. Histological examinations confirmed the completeness of the thyroidectomy by 131I and demonstrated inhibition of spermatogenesis. Aortic atheromatosis was often minimal but atheromatosis was more severe in the coronary.