The aim of this study was to quantitate vascular endothelial growth factor (VEGF) and estradiol-17 beta in follicular fluid (FF) and serum and to correlate the levels of these substances with oocyte maturation. Transvaginal ultrasound-guided follicular aspirations were performed with the purpose of oocyte collection when the follicle was >35 mm in diameter. The mares were divided into 2 groups according to age, and the changes in VEGF and estradiol-17 beta in follicular fluid and serum were examined. Levels of estradiol-17 beta in the follicular fluid and serum of young mares (4-10 years old) did not differ from those in old mares (20-24 years old) (P > 0.05). The follicular fluid and serum concentrations of VEGF in old mares were lower (P < 0.001) than those in young mares. The VEGF levels in young mares did not differ in cases when either mature or intermediate oocytes were collected, whereas its levels in old mares were higher (P < 0.01) when mature oocytes were collected than when intermediate and immature oocytes were collected. Our data suggest that VEGF may play an important role in follicular growth and development, that VEGF levels appear to be age-dependent, and that VEGF levels might be valuable biochemical markers of oocyte maturation.