The aim of this study was to compare the changes in hematological and biochemical parameters in race horses subjected to standardized field exercise tests. The tests were performed two times: in May, at the beginning of the racing season and then four months later. Ten horses were studied in May (5 Thoroughbreds and 5 Purebred Arabian horses). Eight horses were studied in September (4 Thoroughbreds and 4 Arabian horses). For each horse, venous blood samples were collected at rest, after warm-up, just after the end of each gallop, and 30 minutes after the end of the exercise. The concentration of blood lactic acid (LA) hemoglobin (Hb), plasma glucose (Glc), triacylglycerols (TG), glycerol, uric acid (UA), total protein, total thiobarbituric acid-reactive substances (TBARS), the activity of lactate dehydrogenase (LDH), kreatine kinase (CK) and aspartate aminotransferase (AST) and the total antioxidant status (TAS) were determined. The standardized field exercise test performed in September, in comparison to the first test, evoked the lowest increase in blood LA and plasma Glc and TG levels determined during exercise. The rapid return of elevated levels of LA, Hb and UA to their initial values was also observed in September, as compared to data obtained in May