At 2 and 5 mins after an 800-m gallop, venous blood was collected from 26 Thoroughbred racehorses for measurement of blood lactate concentration, packed cell volume (PCV) and haemoglobin concentration. In addition, 14 racehorses were given a strenuous submaximal treadmill exercise test. Heart rates during and after exercise at 10 m/sec on a treadmill inclined at 5 degrees were recorded. Blood samples at 2 and 5 mins after exercise were used to measure PCV, blood and plasma lactate and ammonia concentrations. Results of each exercise test were compared with the retrospective performance of horses in races, using Timeform ratings. The results of the field tests were also compared with the performance of each horse in a race 2 days later. There were no significant correlations between any of the measurements taken after the field test and either subsequent race performance or Timeform rating. Heart rate 4 mins after treadmill exercise was significantly correlated with Timeform rating (r = -0.565, P < 0.05). Blood and plasma lactate concentrations 2 and 5 mins after treadmill exercise were all significantly correlated with Timeform. The highest correlations were with blood lactate concentrations 2 and 5 mins after exercise (r = -0.68, P < 0.01). There were no significant correlations between Timeform and heart rate during exercise at 10 m/sec, heart rates at 1, 3 and 5 mins after exercise, PCV and plasma ammonia at 2 and 5 mins, or the differences between lactate concentration in plasma or blood at 2 and 5 mins after exercise.(ABSTRACT TRUNCATED AT 250 WORDS)