Amino acid catabolism in muscle work was investigated relating to energy production. Tracer 1-¹³C-glycine or leucine was administrated to subjects before bicycle ergometer pedaling, which was performed at 50% VO₂max for 90 min. (easy exercise) or 75% VO₂max for 60 min. (hard exercise). Expiratory ¹³CO₂ that occurred from tracer administrated was analyzed by infrared ¹³CO₂ analyzer and cumulative recovery of ¹³C was calculated.
　(1) ¹³C-glycine and leucine oxidation were promoted by the easy and hard exercise in three male subjects (K. G. 22 yrs. runner, J.I. 27 yrs. non-athlete, H.F. 57 yrs. non-athlete). The expiratory cumulative recoveries (%) at 150 min. of ¹³C-glycine in easy exercise were 53, 48 and 41, and they were reduced by half in hard exercise. In ¹³C-leucine administrations, the cumulative recoveries were 55, 84 and 77 in easy exercise, and they were also reduced by half in hard exercise except subject H.F.
　(2) Five male handball players in the university of Tsukuba showed the expiratory cumulative recoveries at 150 min. from ¹³C-leucine 47.4% in easy exercise and 27.2% in hard. But both recoveries reduced by half respectively in the glucose administrations.

DESCENTE SPORTS SCIENCE Vol.5/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE