Two randomly distributed groups of thoroughbred horses were compared during a 12-week period for their cardio-respiratory and metabolic adjustment to strenuous exercise, training and detraining. The horses were trained following the same standardized schedule and were regularly investigated using standardized treadmill exercise tests (SET) of increasing speed. After the first SET and during the whole experimental period, a group of 6 horses received a probiotic (Bioracing) once a day while a group of 5 horses received a placebo. All other conditions were similar for both groups. During each SET, the oxygen uptake, carbon dioxide output, tidal volume (inspired volume), respiratory rate and expired minute volume were obtained using 2 ultrasonic pneumotachographs and a mass spectrometer. All the parameters were the mean of the values calculated during the last 20 s of the SET. Heart rate was continuously measured with a polar horse tester. Venous blood was sampled before and after the test and analyzed for various biochemical parameters. In both groups, training induced significant modification in most of the cardio-respiratory parameters, ie peak oxygen uptake, peak carbon dioxide output, respiratory exchange ratio, ventilation/min to oxygen-uptake ratio and oxygen-uptake to heart-rate ratio. After the 3-week detraining period, most of the values were again similar to the pre-training values in both groups. However, the training-induced modifications of most of the cardio-respiratory parameters occurred earlier and were proportionally greater in the probiotic-treated group than in the control. The respiratory coefficient decreased in the control but not in the treated group. All other parameters changed similarly in both groups. This suggests that Bioracing could modify the physiological effects of training by improving some aerobic metabolic capacities for carbohydrate utilization, but that this effect occurs only during training and not during periods of physical inactivity.