A standardised incremental exercise test was performed by 9 racehorses with idiopathic laryngeal hemiplegia (ILH), 1 horse with maxillary sinus cysts, 1 horse with epiglottic entrapment, 1 horse with a lesion on the vocal folds, and 1 horse with pharyngitis. Two of the horses with ILH were retested after laryngoplasty and ventriculectomy. The findings were compared with those from 20 normal racehorses. Heart rate, plasma lactate concentration, arterial blood gases, stride frequency, oxygen uptake (V̇o2) and carbon dioxide production were assessed during treadmill exercise on a +10% slope. The group of horses with ILH had significantly (P<0.01) lower peak V̇o2 values (136 ± 5 ml/kg/min) than did the normal group (154 ± 3 ml/klg/min). These values represent mean ± sem. Horses with ILH also had significantly higher (P<0.05) arterial carbon dioxide tensions (Paco2) at 10 m/s and lower speeds at a heart rate of 200 bpm (V200) than the normal group. The horse with maxillary sinus cysts had higher Paco2 tension at 10 m/s than normal, and abnormal values for several cardiorespiratory and metabolic indices. Horses with vocal fold lesions, aryepiglottic entrapment and pharyngitis had arterial blood gas and cardiorespiratory indices that were similar to those of normal horses. One horse which underwent corrective surgery for ILH showed improvements in arterial blood gases and cardiorespiratory indices during exercise, while the other horse had values which were the same as, or worse than, values before surgery. We conclude that the measurement of arterial blood gases and cardiorespiratory indices during treadmill exercise is useful in determining the effect on exercise capacity of various upper airway abnormalities in racehorses.