The aim of this study was to investigate the effect of omega-3 supplementation on some haematological parameters like red blood cells (rbc), haemoglobin (hgb), haematocrit (hct), white blood cells (Wbc), neutrophils (neu) and lymphocytes (lym) that may have a direct effect on the performance of athletic horses. Ten regularly trained Standardbred horses (6 geldings and 4 females, 4–5 years old, mean body weight 500 ± 25 kg) were used for the study. they were randomly divided into two groups. The experimental group received an omega-3 dietary supplementation every day for 30 days. The control group received no supplementation. Every 10 days, horses took part in a 1660 metre harness race and blood samples were collected from each horse: one in the morning before race (pre) and one after race (post). The application of one-way analysis of variance for repeated mesures (ANOVA) showed a statistically significant difference due to the race in the two different groups. The results obtained in the present study show a discordant effect of supplementation with omega-3 on rbc, hct and hgb, while omega-3 supplementation has been shown to have a better effect on Wbc, neu and lym, but further studies should be performed to better evaluate the benefits of these on the performance of the athletic horse.