This study examined the scaling relationships of net O₂ uptake [O_2(net) = O₂  resting O₂] to body mass (M_B) and combined mass (M_C = M_B + bicycle) during uphill treadmill bicycling. It was hypothesized that O_2(net) (l/min) would scale proportionally with M_C [i.e., O_2(net)  M^1.0_C] and less than proportionally with M_B [i.e., O_2(net)  M_B]. Twenty-five competitive cyclists [73.9 ± 8.8 and 85.0 ± 9.0 (SD) kg for M_B and M_C, respectively] rode their bicycles on a treadmill at 3.46 m/s and grades of 1.7, 3.5, 5.2, and 7.0% while O₂ was measured. Multiple log-linear regression procedures were applied to the pooled O_2(net) data to determine the exponents for M_C and M_B after statistically controlling for differences in treadmill grade and dynamic friction. The regression models were highly significant (R² = 0.95, P < 0.001). Exponents for M_C (0.99, 95% confidence interval = 0.80-1.18) and M_B (0.89, 95% confidence interval = 0.72-1.07) did not differ significantly from each other or 1.0. It was concluded that the 0.99 M_C exponent was due to gravitational resistance, whereas the M_B exponent was <1.0 because the bicycles were relatively lighter for heavier cyclists.