In the course of a study of the change in plasma volume accompany-ing treatment with desiccated thyroid in myxedema (1), it was noted that after the basal metabolic rate had been restored to normal, further increases in thyroid dosage up to four times the maintenance requirement produced proportionately much smaller increases in basal metabolism than did the maintenance dose. The effect at a normal level, in fact, appeared to be of about the same magnitudeas that produced by giving thyroid to a normal individual. It seemed desirable to confirm these observations, using thyroxin intravenously instead of desiccated thyroid by mouth, in order to eliminate the possibility of non-absorption of the latter from the gastro-intestinal tract.