To investigate the effect of the radiant heat on the human body in a hot environment, the subjects exposed their nude back to a radiant heat of 1.3 and 2.6 cal/ cm2. min, using the exsiccating infrared illuminators under a hot ambient condition of a temperature 31℃, with a relative humidity of 55% and a 0.5 m/sec air flow. The 8 subjects were healthy male college students aged 20 to 25. The following results were obtained by estimating the physiological reactions to different degrees of radiant heat at rest for 60 minutes and during exercise for 30 minutes on a bicycle e1gomete1·by 272 kg. m / min (or 600 kp. m/ min). 1) The mean skin temperature, heart rate, respiration rate and body weight loss r ate increased at rest in parallel with the degree of the radiant heat, and during exercise the mean skin temperature, heart rate, respiration rate, body weight loss rate and respiratory volume increased, but the Na+ loss rate decreased. The regression equation was obtained to show the quantitative relationship between the degree of the radiant heat and the physiological body reactions. 2) By computing the Heat Tolerance Index by Inoue et al., it was clarified that the higher the degree of the radiant heat was, the smaller was the index. And as there was a close correlation between the indices both at rest and during exercise, it was suggested that for the evaluation of heat tolerance, the radiant heat by the infra1ed illuminators is applicable as additional heat loading besides hot water bathing or staying in a hot chamber
