Body mass scaling of peak oxygen uptake in 20- to 79-yr old adults.Med. Sci. Sports Exerc., Vol. 29, No. 12, pp. 1602-1608, 1997. Despite growing evidence in support of the power function ratio (PFR) for body mass (MB) scaling of peak oxygen uptake (˙VO2PEAK), research literature preferentially reports ˙VO2PEAK values scaled by the simple ratio (SR) method. Theory suggests that VO2PEAK should scale with MB to the 0.67 power (i.e., PFR), while SR scaling assumes an MB exponent of 1.0. This study was designed to determine whether statistically derived MB exponents for a heterogenous sample supported PFR or SR scaling of ˙VO2PEAK. Two hundred thirty women (mean± SD: 47.5 ± 16.8 yr and 64.7 ± 11.5 kg) and 210 men(45.6 ± 16.4 yr, 81.77 ± 12.73 kg) between 20 and 79 yr were evaluated using multiple log-linear regression analysis to determine the MB exponent for ˙VO2PEAK after statistically controlling for age, gender, percent body fat, height, and a self-reported physical activity score. The resulting MB exponent was 0.653 (95% CI: 0.530-0.775) after controlling for all five covariates but increased to 0.756 (0.651-0.862) when height was dropped from the model. Both exponents differed significantly from 1.0 (P < 0.001). These results support the use of PFR scaled˙VO2PEAK values in adults.