OBJECTIVE To determine whether racehorses undergoing regular exercise at 2 intensities or stall rest during a period of reduced training (detraining) would differentially maintain their cardiopulmonary and oxygen-transport capacities. ANIMALS 27 Thoroughbreds. PROCEDURES Horses trained on a treadmill for 18 weeks underwent a period of detraining for 12 weeks according to 1 of 3 protocols: cantering at 70% of maximal rate of oxygen consumption ([Formula: see text]o2max) for 3 min/d for 5 d/wk (canter group); walking for 1 h/d for 5 d/wk (walk group); or stall rest (stall group). Standardized treadmill exercise protocols (during which cardiopulmonary and oxygen-transport variables were measured) were performed before and after detraining. RESULTS Mass-specific [Formula: see text]o2max, maximal cardiac output, and maximal cardiac stroke volume of all groups decreased after 12 weeks of detraining with no differences among groups. After detraining, arterial-mixed-venous oxygen concentration difference did not decrease in any group, and maximal heart rate decreased in the walk and stall groups. Run time to exhaustion and speeds eliciting [Formula: see text]o2max and maximal heart rate and at which plasma lactate concentration reached 4mM did not change in the canter group but decreased in the walk and stall groups. CONCLUSIONS AND CLINICAL RELEVANCE Horses following the cantering detraining protocol maintained higher values of several performance variables compared with horses following the walking or stall rest protocols. These results suggested that it may be possible to identify a minimal threshold exercise intensity or protocol during detraining that would promote maintenance of important performance-related variables and minimize reductions in oxygen-transport capacity in horses.