Thyroidectomized rats were injected with triiodothyronine or thyroxine and the magnitude of the response was measured at various time intervals. The heart rate and the lipolytic effect of epinephrine on adipose tissue were increased 3 hr after giving 45 μg of T3 intravenously and at 6 hr after giving 15 or 45 ng of T3 subcutaneously. Five μg of T3 and 60 μg of T4 produced significant effects on the heart rate and the epinephrineinduced lipolysis in adipose tissue at 12 hr. The magnitude of these effects did not increase with the longer intervals of pretreatment, nor did it increase with the higher doses of hormone. Thyroidectomy elevated cardiac glycogen and pretreatment with thyroid hormone reduced it. Body temperature was sometimes increased by 12 hr but no consistent changes in liver glycogen were produced by thyroidectomy or thyroid replacement. The findings suggest that the latent period is shortest for those effects arising from increased sensitivity to catecholamines and that the subsequent alterations in oxidative metabolism may be mediated by these earlier changes. (Endocrinology76: 323, 1965)