Development of the musculoskeletal system occurs, in part, through an impact on regulatory proteins, such as activin A and decorin. Activin A induces atrophic effects on skeletal muscle, and decorin regulates the formation of connective tissue proteins like collagen and elastin in tendons and ligaments. The aim of this study was to evaluate the influence of different intensities of exercise on blood plasma activin A and decorin concentrations in horses. Ten young purebred Arabian horses were subjected to routine training sessions on a racetrack, and another 11 adult horses of the same breed participated in endurance rides. Race horses were examined during gallop tests over a distance of 1200 m and endurance horses at distances from 20 to 80 km. Blood samples were collected at rest and after exercise to determine plasma concentrations of activin A, decorin and cortisol. Despite differences in the intensity of exercise, the plasma decorin concentration remained unchanged, and activin A tended to decrease in endurance horses only. The exercise-induced changes in plasma activin A concentrations correlated with the covered distance (r = -0.43; p<0.05), but not with the changes in cortisol values. Further studies are needed to confirm the usefulness of plasma activin A as a potential indicator of a horse's endurance performance.