Blood was obtained from patients who were divided into four groups: (A) euthyroid, (B) untreated hyperthyroid, (C) iodine-treated hyperthyroid and (D) hypothyroid. The erythrocytes from each patient group were washed and resuspended in Krebs-phosphate solution for study by the Warburg respirometric technic. When the erythrocytes obtained from patient groups B, C and D are compared with group A (control), a +92 per cent, +23 per cent and -15 per cent change is noted, respectively, in their mean QOO2 values when QOO2 values for erythrocytes for each patient in each patient group (excluding group C) are plotted relative to their respective serum PBI concentration (µg. per cent), a curve of exponential order appears to be the curve of best probable fit for these data. The mean QOO2 value for the iodine-treated hyperthyroid group falls on the ordinate of this curve (fig. 1) between the euthyroid and untreated hyperthyroid groups, though the mean serum PBI value (abscissa), of course, has no meaning for this group.