Blood samples were taken from 21 horses before competition (S), after a race (W) and after a 30 min. rest period (R). Five-year-old horses taking part in a championship were compared with older horses starting in the Championship of Poland. The distance for young horses was shorter than the one for older ones. The changes of parameters such as Ht, Hb, LA, Glu and TP in the blood of five-year-old horses were higher than those of older horses. It was found that most of the five-year-old horses starting in the championship had not been well trained for the competition, while the older horses participating in the Championship of Poland were prepared properly for that race.