Daily admn. of intermedin prepns. (from hog pituitary glands) to rats for periods of 2-4 weeks results in decreases of 30-80% in the avg. melanophore hormone content of the pituitary. Decreases of greater magnitude are observed following thyroidectomy and the admn. of thio-uracil. Melanophore hormone levels of the pituitaries of thy-roidectomized animals are gradually restored during periods of 1-3 mos., while the effects of thiouracil persist during treatment and for a considerable period following withdrawal of the drug. Slight transitory increases in melanophore hormone content were observed in animals made hyperthyroid by feeding desiccated thyroid powder.