Doppler ultrasonography is a noninvasive technique which enables us to follow the physiologic and physiopathologic changes in blood flow in tissues. It is becoming an essential tool in veterinary medicine, especially in theriogenology. Twenty-seven Arabian mares were grouped by age ('young', 3-10 y, n = 15; 'old', 19-23 y, n = 12). The uterine arteries of the mares were examined using Doppler ultrasonography when an ovarian follicle >= 35 mm was visible (Day -1). After these measurements, human chorionic gonadotropin (hCG; 2500 IU) was administered to 14 mares selected randomly. One day later (Day 0) Doppler ultrasonography was repeated and then the dominant follicles were aspirated to collect follicular fluid in all groups. On the next day (Day +1), Doppler indices of the uterine artery blood flow were measured again. Blood samples were also collected just prior to ultrasonography, for measuring serum estradiol and progesterone levels. We found that preovulatory hCG administration had no significant effects on uterine artery blood flow indices, or serum or follicular fluid estradiol concentrations. The uterine artery resistance index might decrease in young mares after ovulation, possibly because of increased uterine perfusion. (C) 2014 Elsevier B.V. All rights reserved.