Percentage of heart rate reserve (%HRR) is widely considered to be equivalent to % of maximal oxygen consumption (%VO2max) for exercise prescription purposes. However, this relationship has not been established in the literature, and a theoretically stronger case can be made for an equivalency between %HRR and %VO2 Reserve (%VO2R) (i.e., the difference between resting and maximal VO2). The current study hypothesized that %HRR is equivalent to %VO2R, not %VO2max, and that the discrepancy between %HRR and %VO2max would be inversely proportional to fitness level. Sixty-three adults performed incremental maximal exercise tests on an electrically braked cycle ergometer. HR and VO2 at rest, at the end of each stage of exercise, and at maximum were used to perform linear regressions on %HRR versus %VO2max, and %HRR versus %VO2R for each subject. For %HRR versus %VO2max, the mean intercept and slope were -11.6 +/- 1.0 and 1.12 +/- 0.01, respectively, which were significantly different (P < 0.001) from 0 and 1, respectively. For %HRR versus %VO2R, the mean intercept and slope were -0.1 +/- 0.6 and 1.00 +/- 0.01, respectively, which were not distinguishable from the line of identity. There was a significant (P < 0.01) inverse relationship between fitness level (VO2max) and the discrepancy between %HRR and %VO2max. In conclusion, %HRR should not be considered equivalent to %VO2max. Rather, %HRR is equivalent to %VO2R, and this relationship should be used in exercise prescription.