The in vitro metabolism of glycerol by epididymal fat pads from thyroidectomized rats daily injected with either 0, 0.1 or 1.8 μg of L-thyroxine/100 g body wt. was compared with that from intact controls. The basal as well as the adrenalin- or glucose-enhanced release of glycerol to the medium were similar in the tissues from all the groups. The effect of insulin in decreasing the lipolytic action of adrenalin was greater in the thyroidectomized animals treated with either O or 0.1 μg of thyroxine than in the other two groups. The utilization of p-1 4c]glycerol for the formation of CO2 and glyceride glycerol was increased in the thyroidectomized rats; this effect was smaller when the animals were treated with 0.1 μg of thyroxine and disappeared when they were treated with 1.8 μg. In the presence of glucose the difference in utilization of glycerol between the groups disappeared. The formation of fatty acids from glycerol was greater in the presence than in the absence of glucose and was similar in the hypothyroid animals and the controls. The effect of adrenalin in decreasing the utilization of [1-1 4C]glycerol was less in the tissues from hypothyroid rats than in the controls. The decrease of the action of adrenalin by insulin in the tissues from thyroidectomized rats treated with O or 0.1 μg of thyroxine was greater than in the controls. The increased capacity to form glyceride glycerol from glycerol in tissues from hypothyroid animals contributes to the high re-esterification of fatty acids described for these animals. The effect of glucose is explained in terms of its competition with glycerol for the synthesis of a-glycerophosphate.