THIS PAPER is a summary of the experiments carried out in the rat in order to determine: a) the effect of thyroidectomy on the diabetes produced by alloxan or by ample pancreatectomy (extirpation of 95% of the pancreas); b) the effect of hyperthyroidism on the same diabetic states. Some of these points have been studied by Houssay and Foglia, Prieto Diaz and Sara (1945), Houssay and Sara (1945), and Martinez (1945, 1946). THE THYROID GLAND AND SUSCEPTIBILITY TO ALLOXAN This was studied in white rats of the male sex ranging in weight from 100 to 200 grams, which were divided into 5 groups: a) the first group received powdered thyroid in a daily dose of 10 to 20 mg. per 100 grams of body weight for 20-29 days; b) the second group was treated similarly for 60 days; c) the third group was thyroidectomized 20 to 29 days before; d) the fourth group was treated with thiouracil in daily doses of 20 mg/100 grams body weight for 30 days; e) finally, the fifth group acted as control. All animals were fed the usual diet of our Institute which consists of: wheat 3 kg.; corn, 2 kg.; white bread, 5 kg.; milk, 14 litres, supplemented by yeast, lucern and cod liver oil.