Whether adaptations in locomotor economy occur across an individual’s life span remains unclear. To understand better the scope of developmental plasticity of locomotor economy, we increasing distal limb mass experimentally by a factor of ~ 3x over the maturation period in growing guinea fowl. At 1 wk, animals were assigned to a control group (CON, n=6) and an experimental group (EXP, n=6) with a mass equal to 3.75% of the individual’s body mass chronically added to the right leg throughout growth. We hypothesized that the EXE group would be more economical at carrying extra limb mass compared to the CON group. We also hypothesized that the EXE group would have a worse economy when walking unloaded compared to CON. At 16 wks of age, a flow-through metabolic system was used to measure metabolic power during treadmill walking at 0.5 m/s. In agreement with our first hypothesis, average net metabolic power in the limb-loaded condition was on average 26% lower in the EXP group (3.9 +- 0.5 W/kg) compared to the CON group (5.3 +- 1.1 W/kg) (p = 0.03). However, contrary to our second hypothesis, the net metabolic power was not different between the EXE and CON in the unloaded condition. Surprisingly, metabolic power was also not different between the loaded and unloaded condition in the EXE group, despite the presumably large reduction in mechanical work associated with lifting and accelerating the limb after removing the mass. These preliminary data suggest that locomotor economy may be affected by loading history during growth and may be tuned to the animal’s habitual loading environment. Supported through NIH Grant R21AR071588.