The purpose of this study was to compare the aerobic capacity during swimming and running from the view point of maximal oxygen uptake (VO2max) and anaerobic threshold (AnT). Fifteen subjects (9 swimmers and 6 field athletes) performed progressive exercises until exhaustion by use of a treadmill and an experimental swimming pool. Respiratory responses were measurered using a computerized breath-by-breath test system for the determination of AnT and VO2max, and the time points where they occurred. The results were as follows ; (1) Between the two exercises, there were significant differences in fractional concentrations of end-tidal O2 (FETO2), CO2 (FETCO2) and pulmonary ventilation (VE) at the two time points. (2) On the interaction of two exercises and two exercise groups, there were significant differences in FETO2 and FETCO2 at the two time points, and in oxygen uptake (VO2) and VO2/wt at the AnT.