Ten healthy sedentary Thoroughbreds with previous race training experience were trained conventionally for 9 weeks. Muscle biopsy samples were obtained before and after training and after 6 weeks of detraining pasture rest. Biopsy samples were obtained from the right deltoid, triceps, vastus lateralis, middle gluteal, biceps femoris, and semitendinosus muscles. The deep-frozen biopsy samples were analyzed for activities of succinate dehydrogenase (SDH), 3-hydroxy-acylcoenzyme A dehydrogenase (HAD), and phosphorylase (PHOS) and for glycogen concentration. The triceps and gluteal muscle samples were also serially sectioned and stained for myofibrillar actomyosin adenosine triphosphatase (ATPase) activity after alkaline (pH 10.3) and sequential acidic (pH 4.34) ATPase inactivation. Fiber types I (alkaline preincubation), IIA1, IIA2, and IIA3 (sequential acidic preincubation over 5 minutes) were identified and were evaluated for fiber-type distribution and fiber areas. Increases in response to training were observed in deltoid and vastus muscle SDH and gluteal muscle HAD activities, and deltoid muscle glycogen concentration (P < 0.05 to P < 0.01). Changes in PHOS activity were not observed. Type-IIA1, -IIA2, and -IIA3 fiber areas in triceps muscle were increased in response to training (P < 0.05 to P < 0.01). Changes in fiber-type distribution did not occur in response to training. Changes in muscle enzyme activities, glycogen concentration, fiber types, and fiber areas were not seen from posttraining to detraining. Further increases were observed when detraining values were compared with pretraining values in deltoid, triceps, vastus, gluteal, and biceps femoris muscle SDH activities and in gluteal muscle glycogen concentration (P < 0.05 to P < 0.01). It was concluded that the predominant failure to detect training-induced muscle enzyme changes, along with documentation of increases in fast-twitch muscle fiber areas, indicate that conventional Thoroughbred training is principally of a sprinting nature. A greater emphasis on longer, slow endurance work early in training might add greatly to Thoroughbred horses' abilities to withstand the rigors of sprint training.