To establish the protocol of a standardized exercise test for evaluating exercise intolerance and degree of fitness in Thoroughbred racehorses, we examined serum lactate concentrations related to exercise intensities using the high speed tradmill. Twelve clinically healthy Thoroughbred racehorses with or without previous training or racing history were assigned to two groups, fit and unfit group, respectively. The protocol used for the standardized exercise test was consisted of two stages:stage ofwarm-up and that of acceleration. During the warm-up, the horses exercised 5 min at 1.8m/s and 3 min 3.4m/s without inclination. At the acceleration stage, exercise test was performed at 10% slope and the speed was increased from the initial 5m/s to the maximal speed which each tested horse could keep up with. The speed was increased with incremental steps of 1 m/s every minute. During the last 15 sec of each step, blood samples were collected for serum lactate determination. V max (masimal treadmill speed which tested horses could keep up with) of the fit group (10.93+_0.33m/s, mean+_SE, n=6) was higher than that of the unfit group (9.52+_0.23m/s, mean+_SE, n=6). Serum lactate concentrations increased exponentially according to exercise intensities. V la4 (speed producing a serum lactate concentration of 4mmol/l) of the fit group, 6.45+_0.26m/s, was higher than that of the unfit group, 5.45+_0.23m/s. La peak (peak plasma lactate concentration during the exercise test) was lower in the fit group (20.34+_1.62mmol/l at 1 min after maximal intensity exercise) than in the unfit group (24.78+_1.09mmol/l at 2 min after maximal exercise step). t50% (time required for the recovery of lactate concentration to be one-half of La peak after maximal exercise) of the unfit group and the fit group were 40.0 and 18.0 min, respectively. Therefore, the protocol of the incremental standardized exercise test utilized in this study seems to be reliable for the assessment of fitness and exercise intolerance for the Thoroughbred racehorses.