The studies were conducted in the studs of Arabian horses including one-year-old fillies that were tested again as breeding mares at the age of six years (77 individuals). The level of irritability was assessed by Excitability Test according to Budzyński's method in three stages: optic (O), accoustic (A), and optic-accoustic one (OA). The apparatus HIPPOCARD – Polar Sport Tester PEH 4000 was used to measure the heart rate in every horse. Assessment of nervous irritability of Arabian fillies at the age of one year by Excitability Test and regression equation can be useful to evaluate the irritability level of mares at the age of six years.