To determine whether age affects cycling efficiency and the energy cost of walking (Cw), 190 healthy adults, ages 18-81 yr, cycled on an ergometer at 50 W and walked on a treadmill at 1.34 m/s. Ventilation and gas exchange at rest and during exercise were used to calculate net Cw and net efficiency of cycling. Compared to the 18-40 yr age group (2.17 ± 0.33 J/kg/m), net Cw was not different in the 60-64 yr (2.20 ± 0.40 J/kg/m) and 65-69 yr (2.20 ± 0.28 J/kg/m) age groups, but was significantly (P < 0.03) higher in the >70 yr (2.37 ± 0.33 J/kg/m) age group. For subjects >60 yr, net Cw was significantly correlated with age (R2 = 0.123; P = 0.002). Cycling net efficiency was not different between 18-40 yr (23.5 ± 2.9%), 60-64 yr (24.5 ± 3.6%), 65-69 yr (23.3 ± 3.6%) and >70 yr (24.7 ± 2.7%) age groups. Repeat tests on a subset of subjects (walking, n = 43; cycling, n = 37) demonstrated high test-retest reliability (intraclass correlation coefficients, ICC, 0.74-0.86) for all energy outcome measures except cycling net energy expenditure (ICC = 0.54) and net efficiency (ICC = 0.50). Coefficients of variation for all variables ranged from 3.1-7.7%. Considerable individual variation in Cw and efficiency was evident, with a ~2-fold difference between the least and most economical/efficient subjects. We conclude that, between 18-81 yr, net Cw was only higher for ages >70 yr, and that cycling net efficiency was not different across age groups.