The aim of this study was to determine the daily rhythms in the blood serum of homocysteine in horses. Ten thoroughbred horses, five athletic (trained for 1 h, 6 days a week) and five sedentary, were used. Blood samples were collected on each subject every 4 h for two days by means of the jugular vein. On each individual sample, the serum concentration of homocysteine was assessed. The results obtained during the experimental period indicated the existence of a daily rhythm of serum homocysteine in sedentary and athletic horses. They also demonstrated that in horses, physical exercise influences the daily rhythm of serum homocysteine.