The metabolic cost of walking is partially determined by the muscle force needed to support body weight and the muscle work needed to redirect and accelerate the center of mass (CoM). During level-ground walking, ~28% of the net metabolic power (NMP) is due to supporting body weight and ~45% is due to performing CoM work. We hypothesized that supporting body weight would incur a greater percentage of NMP for uphill and a lower percentage of NMP for downhill compared to level-ground walking. Additionally, we hypothesized that performing work incur a greater percentage of NMP for uphill and a lower percentage of NMP for downhill compared to level-ground walking. We independently varied weight and mass and measured the metabolic cost of walking at 1.25 m/s on slopes of 0°, ±3°, and ±6° in 10 subjects (5 F). By calculating the metabolic power per newton of reduced body weight for each slope, we found that the percentage of NMP to support body weight was 63.1% and 70.7% on +3° and +6°, respectively, which were greater than 15% for level-ground walking (p<0.025). We found that the percentage of NMP to redirect the CoM was 18.9% and 23.1% on slopes of +3° and +6°, respectively, which were lower than 35% for level-ground walking (p<0.025). Our findings partially support our hypotheses, and elucidate that the percentage of NMP attributed to body weight support and mass redirection are different for slopes;​ compared to level ground, which informs biomimetic assistive device designs aimed at reducing metabolic demand.