Physical effort is one of the key aspects of keeping horses in good condition. The condition of the animal is reflected by multiple blood parameters. The newly discovered cytokine–apelin can pose one of them, however, so far, has not been studied in the horse population. Apelin is produced by adipocytes and myocytes and plays an important role in the energy metabolism of the body through the influence, for example, on the process of adipogenesis and lipolysis. The aim of this study was to investigate if physical effort of various intensity affects the plasma concentration of apelin in horses. The study involved 20 purebred Arabian horses divided into two groups. The first group included 10 race horses, aged 3 years, and second group included 10 horses aged from 6 to 12 years, used in endurance rides. Blood samples were collected from each horse at rest and after exercise. The concentrations of apelin, lactic acid, cortisol, uric acid (UA), triacylglycerols, total plasma protein, and glucose were determined in plasma samples. Race training sessions induced significant decreases in plasma apelin concentrations (P <.05). In endurance horses, significant correlations were found only between exercise-induced changes of apelin and cortisol (r = 0.55) and apelin and UA (r = 0.67) concentrations. In conclusion, the concentration of apelin in equine plasma decreases in response to short-time exercise unlike endurance exercise. Blood plasma apelin concentration determined at rest is lower in long-time regularly trained horses than in the ones at the beginning of their training process.