Size, morphology and motor skills change dramatically during growth and this probably has an effect on the cost of locomotion. In this study, the effects of age and speed on the energy expended while walking were determined during growth. The rate of oxygen consumption and carbon dioxide production were measured in 3- to 12-year-old children and in adults while standing and walking at different speeds from 0.5 m x s(-1) to near their maximum aerobic walking speed. Standing energy expenditure rate decreases with age from 3.42 +/- 0.48 W x kg(-1) (mean +/- SD, n = 6) in the 3- to 4-year-olds to 1.95 +/- 0.22 W x kg(-1) (n = 6) in young adults. At all ages the gross cost of transport has a minimum which decreases from 5.9 J x kg(-1) x m(-1) in 3- to 4-year-olds to 3.6 J x kg(-1) x m(-1) after 10 years of age. The speed at which this minimum occurs increases from 1.2 m x s(-1) to 1.5 m x s(-1) over the same age range. At low and intermediate walking speeds the net cost of transport is similar in children and adults (about 2 J x kg(-1) x m(-1)). In young children walking at their highest speeds the net cost of transport is 70% (3- to 4-year-olds) to 40% (5- to 6-year-olds) greater than in adults.