The objective of the study was to examine the validity of v(4) [velocity run under the defined conditions inducing 4 mmol/L of blood lactate concentration ([LA])] and v(200) (velocity run under the defined conditions inducing a heart rate of 200 beats/min) to differentiate performance level among Standardbred racehorses. For this purpose, 19 Standardbred trotting racehorses with differing racing time records in 2 training yards were submitted to a standardized exercise test to determine their v(4) and v(200) (6 horses of one yard only). The test consisted of 4 or more consecutive intervals depending on when the blood [LA] of a horse increased above 4 mmol/L. Speed and time trotted in each interval as well as time between consecutive intervals were the same for horses of a training yard. The blood [LA] measured after each interval was plotted exponentially against running speed to derive v(4) from the blood lactate-running speed relationship, and the mean heart rate during the intervals was plotted linearly against running speed to derive v(200) from the heart rate-running speed relationship. The correlation coefficient between v(4) and the racing time record was 0.77 and 0.75 for horses in racing yard A and B, respectively. There was no correlation between v(200) and the racing time record. Therefore, v(4) is a valid indicator of performance level of Standardbred trotting racehorses; however, v(200) may not be or to a lesser extent.