Objective:The current study is an endeavor for profound understanding hormonal and biochemical constituents in Arabian mare serum during the estrous cycle.Methods:Ten Arab brood mares of previous foaling records were scanned with ultrasound each other day, and blood samples were collected with each ultrasound examination. The diameter of follicles and corpus luteum was measured. The follicles were classified into small, medium, dominant, 1st and 2nd largest according to their diameter. Hormonal concentrations of thyroxin, progesterone, estradiol and testosterone were assayed. Superoxide dismutase (SOD), malondialdehyde (MDA), nitric oxide (NO) and total antioxidants concentrations (TAC) were measured. Besides, the levels of total proteins, albumin, globulin, glucose, total cholesterol, and triglycerides were assayed. Results:The data revealed that multiple follicles underwent progressive enlargement (≤30 mm). Only the largest (dominant) follicle reached a maximal diameter of (35.82±1.57) mm during estrous phase. During the luteal phase, the corpus luteum reached a maximum diameter of (31.78±1.4) mm. The serum progesterone levels in mare were significantly (P<0.05) higher in luteal phase than those recorded at time of estrus. The reverse was true for estradiol. However, the levels of testosterone and thyroxin did not significantly change during estrous cycle in mare. No significant difference was observed in the serum levels of total protein, albumin and globulin in both phases of estrous cycle. Similar finding was observed in SOD concentrations. In contrast, the concentrations of glucose, cholesterol and triglyceride tended to be significantly (P<0.05) higher at estrous stage than those recorded at luteal one. The same finding was observed in NO levels. On other hand, TAC significantly increased (P<0.05) in mare serum obtained from luteal phase than those obtained at estrous one. A reverse was true for MDA levels.Conlcusions:The steroid hormones and metabolic constituents of mare serum (glucose, cholesterol and triglyceride) as well as NO, MDA and TAC vary according to the stage of the estrous cycle, suggesting their possible role in the process of follicular development in mare.