The concentrations of total and respirable airborne endotoxin in the breathing zone of a pony in 3 different management systems, on 8 occasions, are reported. Airborne endotoxin concentrations in all 3 systems were lower than those reported for many other agricultural environments. However, total airborne endotoxin concentrations in many of the conventional stables exceeded those which can induce pulmonary inflammation and bronchial hyper-responsiveness in normal human subjects, and exceeded those which can induce bronchoconstriction in humans with pre-existing pulmonary inflammation. Therefore, airborne endotoxin may contribute to the development of airway inflammation and dysfunction in conventionally stabled horses. Potentially detrimental effects of airborne endotoxins on the welfare and exercise performance of stabled horses can be reduced by maintaining horses in 'low dust' stables or at pasture, since these environments had significantly lower airborne dust and endotoxin concentrations.