Seventy-two normal children aged between three and 12 years were assessed to establish their walking function. Speed of walking and heart rate were monitored continuously, both barefoot and in shoes, over eight consecutive lengths of a 25m walkway. Since heart rate is affected by speed, the physiological cost index (PCI)--which combines the two parameters--was calculated for each child, both barefoot and in shoes. Speed tends to increase and walking heart rate to decrease with age. However, resting heart rate also decreases with age, so the PCI has the same range for all ages. The mean PCI in shoes was 0.4 and barefoot was 0.38. The PCI for children with pathological gait is appreciably greater than that for normal children, so the PCI is a valuable quantitative indicator of the level of handicap.