The relation between the sweat iodine loss and the urinary iodine excretion was studied on 13 physically active male athletes. There were no statistical differences between the sweat iodine concentrations before and after high iodine food intake, no difference between those before and after 1-h strenuous exercise, or no difference between those of the goitrous and the non-goitrous. These results indicate that the sweat iodine concentration is quite steady and the total iodine loss per day seems to be directly related to the volume of the. sweat lost in each subject. The concentration variation of sweat iodine among individuals is small. The mean concentration of 13 athletes during 8 consecutive days was about 37 ±6.6 μg/l. By comparing the iodine concentration of urine and sweat, the iodine concentration of sweat seems to be more stable than that of urine. The results indicated that the urinary iodine excretion plays a regularoty role for iodine balance in the body and that iodine appeared to be an essential constituent of sweat.