Objective:To evaluate the changes occur in the vagina, cervix and endometrium during the pre- ovulatory period in Arabian mares by using endoscope as a diagnostic tool as well as the availability of taking an endoscopic biopsy during this stage.Methods:Ten cyclic and healthy arabian mares were subjected to ultrasound scanning of their ovaries to estimate the diameter of the preovulatory follicles, collection of blood samples for estimating the hormonal levels (Estradiol-17β and progestrone) for confirming the estrous phase, endoscopic monitoring their vagina, cervix and endometrium, the availability of taking biopsical samples and histological examination of the obtained samples.Results:The obtained data revealed that, the mean diameter of the preovulatory follicles was (3.95±0.34) cm, the plasma estradiol and progestrone levels were (85.98±5.26) pg/mL and (0.73±0.23) ng/mL, respectively, endoscopic monitoring the vagina, cervix and endometrium revealed hyperemic vaginal mucous membrane, soft and opened cervical ostium with abundant estrous mucous and folding, hyperemic and edematous appearance of the endometrium, respectively and the histological features of the biopsical endometrial samples revealed that, The lamina propria-sub mucosa was packed with simple or branched tubular glands lined with simple columnar epithelium showing oval or rounded dark nuclei with different degrees of secretory activities of the glands.Conclusions: The endoscope proved to be a reliable tool for confirming the estrous phase in mares and monitoring the gross features of the vagina, the cervix and the endometrium, in addition to the availability of taking biopsical smears.