The aim of the present study was to test the hypothesis that glutathione peroxidase, uric acid, and a number of hematological biomarkers do not vary in adult mares, over a 24-hour period. Seven adult (age 10 ± 6 yrs; weight 370 ± 30 kg) Arabian Purebred mares were used. Blood samples were collected every two hours, except during the period after-meals, when samples were collected every 30 minutes, totaling four samples in two hours. These samples were used to analyze glutathione peroxidase (GPx), uric acid (UA), glucose (GLU), total plasma protein (TPP), red blood cells (RBC), hemoglobin (HB), hematocrit (HT), red cell distribution width (RDW), white blood cells (WBC) and lymphocytes (LYM). One-way repeated measures analysis of variance (ANOVA) was used to determine significant differences. Tukey’s test was used for multiple comparisons between the averages. Ρ values < 0.05 were considered statistically significant. Antioxidant biomarkers GPx and UA, as well as hematological biomarkers TPP, RBC, HB and HT exhibited a diurnal rhythm that was not affected by food ingestion, while RDW-SD, RDW-CV, WBC and LYM did not present a statistically significant change in the same period.