The basal metabolic rate of 21 Japanese infants (2-4 years of age) was measured 3-5 times over in each season for a year by the helmet-mask method, and the seasonal changes of those were as follows: (1) The basal metabolic rate of infants was the lowest in summer and the highest in winter, that of boys in winter was increased by 13.1%, in spring, +8.5%, and in autumn, +3.6% of that in summer; that of girls in winter being increased by +13.5%, in spring, +11.2%, and in autumn, +5.6% of that in summer. (2) The amounts of O₂-consumption were in proportion to the basal metabolic rate, and were the highest in winter, and the lowest in summer. But the amounts of CO₂-excretion were not different in each season. (3) The respiratory quotient showed high value in summer and autumn, and the lowest in winter.