The relationship between gait and the respiratory response to exercise was examined in five standardbred racehorses which exercised on a treadmill at a pace and a gallop. After an initial warm-up, respiratory rate and stride frequency were measured after one and two minutes of treadmill exercise at 80 per cent of maximal oxygen consumption (VO2max), after one minute at 100 per cent VO2max and after two minutes at 100 per cent VO2max (galloping horses only). Exercise at 100 per cent VO2max continued until the horses showed signs of fatigue. Arterial blood was collected during exercise and when they were fatigued for the measurement of oxygen and carbon dioxide tensions, haemoglobin saturation and pH. Venous blood temperature was also recorded. The mean (SE) time to signs of fatigue was significantly (P < 0.05) less in the pacing horses (7.2 [0.4] minutes) than in the galloping horses (8.0 [0.4] minutes). The mean (SE) resting PCO2 was 47.7 (1.9) torr. During the pacing and galloping exercises at 80 per cent and 100 per cent VO2max the PCO2 remained in the range of 41.1 to 66.8 torr, despite concurrent hyperthermia and acidosis. The PCO2 during exercise was not significantly dependent on gait or exercise intensity. The PO2 was significantly higher in pacing horses during exercise at 80 per cent VO2max (111 [7] vs 96 [6] torr). The mean (SE) arterial blood pH decreased from 7.428 (0.025) during pacing at 5 m sec-1 to 7.250 (0.042) when the horses were fatigued.(ABSTRACT TRUNCATED AT 250 WORDS)