Physical effort influences free glycerol concentration in the blood of horses, and also glycerol bounded in triacylglycerols. The aim of this study was to evaluate the influence of the intensity of exercise that was measured by blood lactic acid (LA) level on plasma glycerol concentration in Purebred Arabian horses. During the training sessions of 36 Arabian horses (26 stallions 3–6 years old and 10 mares aged 3–4 years), 53 exercise tests were performed. Blood samples were taken at rest, at the end of the training session composed by 1200 meters gallop at racetrack with a speed 8–13 m/s, and thirty minutes later. Blood LA and plasma glycerol concentrations were determined by enzymatic tests. The post-exercise LA value correlated positively with the concentration of glycerol that was determined at the same time (r = 0.63) and 30 minutes later (r = 0.80).