1. The relation of oxygen intake and walking speed has been investigated in a sixty-two-year-old patient before and after unilateral hip replacement for osteoarthritis.

2. Before operation walking speed was reduced to three to five kilometres per hour and stride length to 73 centimetres. Oxygen intake was 0·9-l·2 litres per minute and net energy cost 49 kilocalories per kilometre.

3. Nine months after operation the patient could walk at up to 8 kilometres per hour with a stride of 115 centimetres, oxygen intake of 2·4 litres per minute and net energy cost of 34 kilocalories per kilometre. Most of this improvement took place within three months of operation.

4. The oxygen intake of walking at ordinary speeds after full recovery was comparable with that of control subjects aged twenty-seven to twenty-nine years. In very fast walking, however, the oxygen intake was higher than that of the controls, whose oxygen intake was 1·95 litres per minute at 8 kilometres per hour.

5. The use of a stick or crutches did not reduce the oxygen intake in slow walking. However, crutches enabled the subject to walk at over 8 kilometres per hour with oxygen intake of 2·4 litres per minute (85 per cent of maximum oxygen intake).