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SUMMARY To evaluate normal cardiorespiratory and metabolic responses of Thoroughbred horses to a standardised treadmill exercise test, we examined 28 horses ranging in age from 1 to 4 years. The group consisted of eight yearlings, eight 2-year-olds and twelve 3 and 4-year-olds. All horses except the yearlings were in training, and either racing or ready to race, at the time of examination. None of the horses had histories of performance problems. On the first day the horses received a full physical examination, resting electrocardiogram, upper respiratory tract endoscopy and either one or two acclimatisation runs on the treadmill. The following day they were given an exercise test on a treadmill inclined at 6 degrees (+10% slope). The test consisted of 3 min at 4 m/sec, 90 sec at 6 m/sec and 60 sec intervals at 8, 10, 11, 12 and 13 m/sec. During the last 15 sec of each step, blood samples were collected for plasma lactate determination, expired respiratory gases were obtained using an open flow mask system for measurement of oxygen uptake, and heart rate was measured using telemetry electrocardiogram. From these measurements, various derived values were calculated, which have been used by others as indices of exercise capacity. These values included: V200 (speed at HR of 200 bpm), VHrmax (speed at which horses reached maximum HR), VO2-200 (oxygen uptake at a HR of 200 bpm), VO2max (maximum oxygen uptake), VLA4 (speed at which horses reached a plasma lactate of 4 mmol/I) and HRLA4 (HR at which horses reached a plasma lactate of 4 mmolll). The yearlings had significantly lower values than the older age groups for most of the derived values. However, the slope of the VO2 versus speed regression line between 4 and 8 m/sec was not different to those of the older groups. The 2-year-olds had significantly lower VO2-200 and V200 values than the 3- and 4-year:old horses. Overall, there were strong positive correlations between V200 and VO2-200, V200 and VO2max, VO2max and VO2-200. None of the horses had abnormalities on physical, endoscopic or electrocardiographic examination. This study provides some normal data for horses during incremental treadmill exercise that may assist in the evaluation of horses presented for poor racing performance.