Objective - To determine the effects of 3 rations (low grain, fat, high grain) on plasma creatine kinase (CK) activity and lactate concentration in Thoroughbred horses with recurrent exertional rhabdomyolysis (RER). Animals - 5 Thoroughbreds with RER and 3 healthy Thoroughbreds (control horses). Procedures - Rations were formulated to meet (low-grain and fat rations) or exceed (high-grain ration) daily energy requirements. Each ration was fed to horses in a crossover design for 3 weeks. Horses were exercised on a treadmill Monday through Friday; maximum speed on Monday and Friday was 11 m/s (6% slope), on Tuesday and Thursday was 9 m/s, and on Wednesday was 4.5 m/s. Plasma CK activity and lactate concentration were determined before and after exercise. Results - Horses with RER fed the high-grain ration had significantly greater CK activity and change in CK activity 4 hours after exercise, compared with those fed the low-grain ration. Horses with RER exercised at the trot or canter had significantly greater increases in CK activity, compared with those exercised at the gallop. Plasma lactate concentrations after exercise were similar in control and affected horses. Lactate concentration and CK activity were not correlated in horses with RER. Conclusions and Clinical Relevance - Rations high in grain and formulated to exceed daily energy requirements may increase episodes of rhabdomyolysis in Thoroughbred horses susceptible to RER.