1. Oxygen consumption, respiratory frequency, and the PO2 of expiratory and interclavicular air sac gases were continuously monitored in six female domestic fowl trained to exercise on a treadmill for 10 min periods at normal or elevated air temperatures. 2. At normal temperatures (20 +/− 2 degrees C) the cost of locomotion rose from 0.46 ml O2 kg-1 m-1 at 0-3 km h-1 to 0.77 ml O2 kg-1 m-1 at the maximum speed of 4.3 km h-1. At 32 +/− 2 degrees C, Vo2 increased by as much as 20% compared to normal temperatures. 3. Hyperventilation occurred at all speeds and at both normal and elevated temperatures. End-tidal and interclavicular PO2 increased, in a parallel manner with speed, the latter remaining consistently 6-7 Torr less than the former both at rest and during exercise.

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