A comparison of walking against vertical (gradient) and horizontal (trailing weight) forces was made during steady-rate exercise at 0.250, 500, and 750 kg-m/min with speeds of 3,0, 4.5, and 6.0 km/h. In all cases exponential relationships between energy expenditure (calculated from the steady-rate respiration) and increasing work rate and speed were observed which indicated that muscular efficiency during walking is inversely related to speed and work rate. "Work" (level, unloaded walking as the baseline correction), "delta" (measured work rate as the baseline correction), and "instantaneous" (derived from the equation describing the caloric cost of work) efficiencies were computed. All definitions yielded decreasing efficiencies with increasing work rates. At work rates above 250 kg-m/min the curves describing the relationship between energy expenditure and work rate were parallel for vertical and horizontal forces, indicating equivalent efficiencies in this range. Only the delta and instantaneous definitions accurately described these relationships for vertical and horizontal work. Determinations of combined work loads (gradient plus trailing weight) were made and the energy costs of both types of work found to be additive.