We examined the urinary iodine excretion of sedentary and physically active male university students in order to estimate the iodine intake of Japanese. Iodine excretion in sweat collected during treadmill exercise was also determined in different dietary iodine levels. The mean urinary iodine excretion of 5 sedentary students during 15 consecutive days was 357 micrograms/day (40-3,390). When high-iodine food, i.e. seaweed, was included in meals in only 22% of the total experimental days of 5 subjects, the urinary iodine excretion was high (1,106 micrograms/day, 298-3,390), but was low (153 micrograms/day, 40-441) when seaweeds were not consumed. An unexpectedly low mean urinary iodine excretion of 149 micrograms/day (50-393) was found in 10 rowing club students during 6 consecutive days of their summer training camp, probably being due to iodine losses in sweat; sweat iodine concentrations were about 37 micrograms per liter, regardless of serum and urinary iodine levels modulated by the dietary iodine level. The present data indicate that the iodine intake of Japanese depends on the amount of seaweed consumption and that it is not necessarily as high as expected from the data obtained in the 1960s. Moreover, our findings indicate the importance of taking account of iodine loss in sweat in the evaluation of iodine nutrition for physically active persons working in hot and humid environments.