Basal metabolic rate (BMR) was determined monthly for a period of 2 years in 17 Korean (8 male and 9 female) subjects who live in Seoul, Korea. In addition, several parameters of thyroid function were studied in January, May and November by administering I-131. The average BMR was lowest in May and highest in December in both sexes, the magnitude of annual variation amounting to approximately 7 to 10 Kcal/hr/m². These characteristic seasonal variations were evident even when BMR values were computed per kilogram lean body mass. A1though these changes in BMR over a period of a year seem to be inversely related to overall changes in the average monthly ambient temperature, the average BMR is not in phase with ambient temperature. Thus the lowest BMR was seen in May while the highest ambient temperature was noted in August, and the highest BMR occurred in October through December while the lowest temperature was in January. Neither thyroidal uptake of I-131 nor the protein-bound I-131 conversion ratio showed any apparent seasonal variations. These findings indicate that the seasonal changes in BMR of the Korean are not determined primarily by the level of ambient temperature but by other factoss, possibly seasonal variation of physical activity.