Seventeen women (mean age 31 yr) participated in a training program divided into an initial 9-wk period and a subsequent 52-wk period, during which time 6 continued to exercise and the remainder detrained. Improvements in VO2max were significant (+34%) during the initial 9 wk and small (+5%) for the final 52 wk. Four women who stopped training showed a decrease in VO2max (-10%) during the last phase. During the initial 9 wk, central adaptation was important, with SV showing an increase of 28% at 80% VO2max. Peripheral adaptation (a-v O2 difference) was unchanged. Subjects who trained an additional 52 wk showed a slight drop in SV at submaximal work loads from the initial increase following the first 9 wk. When compared with the initial test the change at 9 wk in peripheral adaptation was a small and nonsignificant rise, followed by a significant increase at 61 wk. Women who are very unfit initially (predicted VO2max of 28 ml/kg-min), apparently adapt to the initial training with a central change followed by a much stronger peripheral adaptation during a longer training program.