Summary Four unfit thoroughbred horses were exercised on a treadmill twice, 5 weeks apart. Exercise consisted of stepwise increments in treadmill speed up to a maximum of 12 m s −1 and then maintained at this speed until the horses were fatigued. Two of the horses were administered phenylbutazone (4.4 mg kg −1) intravenously immediately before the first exercise period and the other two horses immediately before the second exercise period. Clinical observation revealed stiffness of gait and palpable soreness over the lumbar-sacral region in the horses 24 h after the exercise concluded. Mean plasma aspartate aminotransferase and creatine kinase activities and urinary creatinine clearance did not change as a result of exercise. The mean urinary excretion and clearance of hydroxyproline significantly increased in the 24 h following exercise ( P<0.001). A concurrent increase in the urinary excretion of malondialdehyde also occurred ( P<0.001). Prior administration of phenylbutazone did not affect hydroxyproline or malondialdehyde excretion or clearance, nor did it appear to reduce the severity of soreness after exercise. The results indicate that lipid peroxidation and the excretion and clearance of hydroxyproline increase when unfit thoroughbreds are strenuously exercised on a treadmill.