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Background: Mares are the only companion animals simulating women in the large diameter of their follicles. Horses start reproduction at the age of three years, and some of them live for >30 years, so aging influences their reproductive capacity. Mares are sensitive to summer heat stress as they can sweat like humans. Aim: The current work aimed to study the effects of age (young versus senile), season (cold versus hot), and the hormonal treatments during embryo collection on the dominant and subordinate follicular dynamics and hemodynamics and circulating ovarian hormones in embryo donor mares ovulated twice spontaneously before inducing ovulation for flushing embryos. Methods: Spontaneous oestrous cycles were studied for young mares (<10 years; N = 6) or senile (>20 years; N = 5) during months of the cold season (November to April) and hot season (May to August). In young embryo donor mares, oestrous cycles after inducing ovulation and luteolysis were studied using Doppler ultrasound. Estradiol (E2), progesterone (P4), nitric oxide (NO), total cholesterol, and lactate dehydrogenase (LDH) were measured in blood serum. Results: A decrease in the dominant follicle antrum diameter (p > 0.05) and LDH (p = 0.016) was observed after inducing luteolysis in young embryo donor mares. Both human chorionic gonadotropin (hCG) and PGF2a treatments increased dominant follicle area (p = 0.0001), antrum area (p = 0.001), perimeter (p = 0.001), granulosa area (p = 0.0001), cholesterol (p = 0.0001), NO (p = 0.0001), and E2 (p = 0.0001). The dominant follicle area, antrum area, perimeter, color area, granulosa area, LDH, cholesterol, NO, and E2 increased (p = 0.0001) during the oestrous cycles of the hot season, but the circulatory % (p = 0.0001) declined. Senile mares had lower dominant follicle area (p = 0.002), antrum area (p = 0.0001), granulosa area (p > 0.05), LDH (p = 0.001), cholesterol (p = 0.0001), NO (p = 0.0001), and E2 (p = 0.0001) but higher circulatory % (p = 0.0001) and color area % (p = 0.023). The dominant follicle possesses the largest diameter, area, perimeter, granulosa area, and color area but the lowest circulatory % during spontaneous oestrous cycles, after inducing ovulation, or luteolysis with significant effects of the day of the spontaneous oestrous cycles on their dynamics and hemodynamics. Conclusion: During hot months, mares treated with hCG ovulated 24 hours later and prostaglandin-induced luteolysis was followed by new ovulation five days later. Follicles ovulated during the hot months were larger than those ovulated during the cold months and both had nearly the same color area %. Senile mares ovulated follicles with a lower area and antrum area but a higher color area %, so senile mares can be used as embryo or oocyte donors during the hot season. [ABSTRACT FROM AUTHOR]