The aim of the study was to evaluate whether any differences can occur in the reactions of selected horse blood parameters to training in relation to the length of show jumping horse training and horse achievements. The research material consisted of blood and serum collected from 20 horses (aged 4-11 years). The animals were divided into 4 groups of 5 horses each depending on their number of years in show jumping training, their achievements in show jumping contests, and their sport potential. The following parameters were examined in the blood and serum: hematocrit, hemoglobin concentration, red and white blood cells count, granulocytes, platelets, fibrinogen, total plasma proteins, lactate, and creatine kinase activity. Blood samples were collected four times from each horse: before show jumping training, 5 and 30 minutes after training, and 24 hours after training. The training was followed by an increase of monitored hematological indicators, which was a physiology reaction to physical effort; no abnormalities were noticed. Horses with outstanding sport potential from both age groups were characterized by higher initial values of parameters associated with erythrocyte system, i.e., Hb, Ht. At the same time, changes caused by physical effort (an increase of Ht, Hb, and RBC) proceeded more steadily in the case of horses with outstanding sport potential than others. Physical effort caused a short-term increase of granulocytes and platelets. The marked initial concentration of lactate acid was higher than stated in the literature, and in individual cases reached concentrations of 1.1 to 2.2 mmol/l. Creatine kinase increased in activity following the training process but did not increase any further, which indicated that there were no significant injuries in the muscular system. Despite the existing tendencies, the differences between horses with outstanding sport potential and average animals were not statistically significant and therefore did not facilitate predicting the sport potential of horses, but they may be useful in evaluating the appropriateness of the training process.