THE EXPERIMENTS reported in this paper were undertaken to throw light upon the interrelationship between the thyroid and the anterior pituitary glands. Evaluation of the activities of the thyroid and thyrotropic hormones was made upon the basis of observations of the effect of these hormones upon the oxygen con-sumption of guinea pig thyroid in vitro. The oxygen consumption (202) of guinea pig thyroid tissue, when immersed in a medium containing the material to be tested, was measured by means of the Warburg apparatus. Standard technic for this apparatus was employed, that is to say, one hour at 38°C. with 100 agitations per minute. Thyroid glands of guinea pigs were sliced with very fine scissors and distributed among the vessels of a series of manometers in such a fashion that each vessel contained the same number of slices from each individ-ual thyroid gland.