The purpose of this study was to investigate the effect of intensity upon the aerobic work capacity of women through long-term training, when total amount of work was kept approximately constant for each subject. Seven sedentary adult females, aged 23 to 40 years, participated in a 44-week training experiment. They trained on a bicycle ergometer at 60-65% V^^.O_2max during first 13 (15 for 2 subjects) weeks (T60), at 75-80% V^^.O_2max during next 18 (16 for 2 subjects) weeks (T75) and at 90-95% V^^.O_2max during final 13 weeks (T90). The total amount of work and frequency were kept approximately constant for each subject through training. Mean V^^.O_2max, expressed in 1/min, per body weight and per LBM significantly increased during T60 and T90. The increase during T75 was not significant. The final values during each three training period were not necessarily the highest. V^^.O_2max, V^^._E and O_2 pulse, expressed per LBM, showed a significant relationships (partial correlation keeping the effect of age constant) between their initial values and the total gains (%). The final values attained of V^^.O_2max except an obese subject were highly correlated with age (p<0.01). It was concluded that the qualitative aspect of training intensity was essential for the improvement of V^^.O_2max, on the other hand sufficient quantity of total energy output was necessary for the retention of high fitness level. Furthermore, the improvement of V^^.O_2max with long-term and effective training was significantly related with the initial level based on lean body mass, and the attainable level was significantly limited by age.