The energy cost of locomotion of four Iberian pigs was measured in two experiments conducted when the animals averaged 41·3 (se 0·1) kg (first experiment) and 84·1 (se 0·1) kg (second experiment). The heat production of the pigs was determined when standing or walking at a speed of 0·555 m/s on a treadmill enclosed in a confinement-type respiration chamber, on different slopes (-10·5, 0, and +10·5 % in the first experiment, and -5·25, 0 and +10·5 % in the second experiment). The energy costs of locomotion, estimated from the coefficients of linear regressions of heat production per kg body weight (BW) on distance travelled, were in the first experiment 2·99, 3·31 and 5·88 J/kg BW per m for -10·5, 0, and +10·5 % inclines respectively, and 2·56, 2·84 and 7·13 J/kg BW per m for -5·25, 0 and +10·5 % inclines respectively, in the second experiment. The net energy cost of locomotion on the level appeared to be independent of live weight, attaining a value of 2·98 J/kg BW per m. Also, it was found that within experiments the net energy cost of walking on negative slopes was similar to that for locomotion on the level, indicating that no energy was recovered on vertical descent. Mean values were 3·11 and 2·72 kJ/kg BW per m for the light and heavy pigs respectively. The energy cost of raising 1 kg BW one vertical metre was found to be 27·1 J/kg BW per m in the first experiment and 40·0 J/kg BW per m in the second experiment. Correspondingly, the calculated efficiency for upslope locomotion appeared to decline with increasing BW, resulting in average values of 36·2 and 24·5 %.