Hind-limb perfusion was used to study the effect of thyroidectomy on some metabolic parameters in the skeletal muscle of the rat. A week after thyroidectomy obtained by one dose of 3/4 mCi 131I, neither T4 nor T3 was detected in the blood. Lactate production and glycerol production were already decreased a week after the treatment and reached a base level at two weeks. At that time, the oxygen consumption was significantly lower (70% of initial level) than in the control animals and decreased further in the third week to nearly 50% of the control level. Glucose consumption and alanine release were decreased three weeks after thyroidectomy. One dose of T3 (10 microgram/100 g b. w.), administered to animals two weeks after the injection of 131I, restored the oxygen consumption, lactate production, and glycerol production to normal levels in 24 h. After 48 h, the glucose consumption was normal. Glycerol production was already significantly increased 6 h after T3 injection in animals one week after thyroidectomy, and in another group of animals two weeks after thyroidectomy. Apparently the diminished oxygen consumption in the latter group does not retard the lipolytic response to T3. No direct relationship could be found between the activity of lipolytic process and the thyroid hormone controlled oxygen consumption.