ABSTRACT.One hundred and two sedentary and fortysix active adult Japanese females, whose ages ranged from 20‐62 years, were tested for maximal oxygen intake (&OV0312;o2max) on a bicycle ergometer. The active females participated in a regular physical conditioning program. For the sedentary females, the mean &OV0312;o2max's in ml/kg · min were 32.4 for 20‐29 years, 27.9 for 30‐39 years, 26.0 for 40‐49 years and 24.4 for 50‐62 years. For active females, the average &OV0312;o2max values were 37.5 for 20‐29 years, 31.9 for 30‐39 years, 28.8 for 40‐49 years and 27.3 for 50‐59 years. &OV0312;o2max was significantly higher at all ages in the active group (p < 0.05). The rate of decline of &OV0312;o2max as a function of increasing age after the twenties was 0.283 ml/kg · min per year for sedentary females and 0.316 ml/kg · min per year for active females; the respective regression equations being Y = ‐0.283 X + 38.5, and Y = ‐0.316 X + 43.3.