Ventilation, metabolism, arterial blood gases, and blood and cerebrospinal fluid (CSF) acid-base status were measured in exercise studies on seven ponies during mild, moderate, and near-maximal treadmill exercise. CSF and arterial blood were sampled via indwelling catheters. Generally measurements were made during the 3rd, 6th, and 9th minute of steady-state exercise, with CSF sampled only during the 9th minute. Alveolar ventilation (VA) and metabolic rate (VO2) increased proportionately during exercise below the anaerobic threshold, but above this threshold, VA increased at a faster rate than VO2. The similarity of these response to those observed in man suggests the pony is a suitable animal model for study of exercise hyperpnea. No change in CSF acid-base balance occurred with light-to-moderate work; however, with near-maximal work a fall in CSF carbon dioxide partial pressure due to hyperventilation caused CSF to become alkaline (pH = 7.380) relative to rest (pH = 7.330). CSF lactate increased slightly with exercise but had no effect on CSF [HCO3-], which remained constant from rest to severe exercise. We conclude that it is unlikely the hyperpnea at any intensity of exercise results from an increased H+ stimulation at the medullary chemoreceptor.