The aim of the research was to assess the effects of physical activity of various intensity on the degree of damage to certain organs resulting from increased free radical production, as well as the adaptability of the organism to physical exercise. Two groups of healthy 3-5-year-old full-blooded racehorses were assessed. The first one ran a 2400-meter gallop race, which is considered a short-lasting, intense physical activity; lipid status was assessed prior to, and 48 and 72 h after the race. The second group ran a forty-kilometer endurance ride, which is a long-lasting moderate physical activity; the lipid status was assessed immediately before, on finishing and 48, 72, 96, 120 and 144 h after the race. The total activity of LDH changed 72 h and 96 h following the gallop race (p>0.05), whilst the maximum activity was measured immediately after the endurance ride. By means of electrophoresis LDH in all the horses 5 isoforms were detected. The activity of LDH1 72 h after the gallop race significantly rose in comparison to the one before the race (p0.05). Following the endurance ride LDH1 activity rose at all sampling times, reaching the maximum at 96 h and 144 h in comparison to the values both before and on finishing the ride (p<0.01). The increase in the activity of LDH2 was significantly higher 48 h, 72 h, 96 h and 120 h (p<0.05) after the race in comparison to that before the race, and at 48 h, 72 h, 96 h, 120 h and 144 h (p<0.05) in comparison to the values on finishing the race. LDH3 activity significantly decreased and LDH5 rose immediately after the endurance ride (p<0.01), whilst LDH4 significantly rose at all times following the endurance ride (p<0.01). The CK activity pointed to high, medium and low degree of adaptation of horses to physical activity. The concentrations of total proteins, albumins and globulins remained within the physiological range at all sampling times, with the exception of 96 h after the endurance ride, when the fall in albumin concentration was significant (p<0.01). Long-lasting physical activity of low intensity leads to cellular damage in the myocardium, muscles, liver and to hypoalbuminaemia, which is a consequence of free radical production. [Projekat Ministarstva nauke Republike Srbije, br. 173034 i br. 31085]