The aim of the study was to demonstrate that monotonous everyday work lasting for several hours a day and many weeks long in a commercial horse-riding centre may lead to chronic fatigue and consequently to increased susceptibility to oxidative stress in “recreation” horses used there. During the experiment, from the mid-June to the beginning of September, 16 half-bred horses - eleven geldings and five mares at the age of 4-8 years, were kept under uniform environmental conditions in a seasonal horse-riding seaside centre in Poland and used on the average four hours a day for six days each week. Before and after the season, their blood samples were collected, in which haematological indices, Mg, Fe, Cu, Zn and Mg concentrations, and total plasma reducing potential by the FRAP method were determined. The results showed a significant deterioration of peripheral blood morphology at rest and a decrease in plasma reducing potential. The product RBC × HGB × 10¹² kg × dm⁻⁶ decreased from 1.66 ± 0.71 to 0.99 ± 0.16 (Pu < 0.005). FRAP decreased from 5.14 ± 0.88 to 4.16 ± 0.67 × 10⁻⁴ mol/dm³ (Pp < 0.001), while Cu concentration increased to the upper normal range, i.e. to 2.18 ± 0.65 × 10⁻⁵ mol/dm³ (Pp < 0.06). At the same time, Mg, Fe, Zn and Mn concentrations did not change significantly. The decrease of RBC and HGB values was acknowledged by us as a clinical symptom of chronic fatigue which, apart from general lesions such as weight loss, unwillingness to work and growing irritability, was accompanied by thrombocytopenia, hypoalbuminemia, dehydration, disturbance in microelement homeostasis and a drop in total plasma antioxidant potential. Consequently it is concluded that in some cases, at the end of working season, “recreation” horses should not be forced to an intensive effort, e.g. a race or jumps, because during intensive aerobic work both the oxygen transport system and antioxidant defence system undergo an overload, which is followed by the growing probability of damage to tissues by ROS.