Physiological reaction and oxygen intake during exercise and recovery were measured in fourteen young female Japanese during the follicular phase of their menstrual cycle at 25 degree C with 50% relative humidity and at 35 degree C with 50% relative humidity. Subjects, clad in bathing suits only, performed a bicycle ergometer exercise at a constant work load of 600 kg . m/min at a cycling rate of 50 rpm for 20 min and recovered while remaining on the bicycle ergometer for 40 min. The mean values of sweat volume and skin temperature were significantly greater at 35 degree C than at 25 degree C. It has been shown that heart rate and rectal temperature during exercise were slightly higher at 35 degree C than at 25 degree C, while those during recovery were significantly higher at 35 degree C than at 25 degree C. Oxygen intake, oxygen debt, and the fall in diastolic blood pressure after exercise were considerably greater at 35 degree C than at 25 degree C. The increase in oxygen intake in a hot environment might result from an increased metabolism due to higher body temperature and increased energy requirement for heat dissipation such as profuse sweating, higher heart rate, and increased ventilatory volume. The increase in oxygen debt in a hot environment might reflect the increased metabolism caused by higher body temperature and the increased production of lactic acid in the working muscle as a result of an insufficient blood supply to the muscle. The increases in sweat volume, oxygen intake during exercise, and oxygen debt in women in a hot environment were considerably smaller than corresponding values for men. The smaller increase in sweat volume in women in a hot environment could reflect a smaller oxygen intake and a more marked dilation of skin vessels in women than in men.