Changes in the concentrations of steroids and prostaglandin F in preovulatory follicles of the mare after administration of hCG

Fluid was aspirated from the preovulatory follicle of Group 1 mares (N = 6) when follicles reached 32-34 mm in diameter. Group 2 mares each received an i.v. injection of hCG when the preovulatory follicle reached 35 mm. Aspiration of follicular fluid was performed 28-32 h after treatment. Follicular fluid was aspirated from Group 3 mares 28-32 h after the preovulatory follicle reached 35 mm in diameter. Concentrations of progesterone were significantly higher in follicular fluid from Group 2 mares than in that from mares in Groups 1 and 3. Testosterone was significantly higher in follicular fluid from Groups 2 and 3 than in Group 1 mares. There were no significant differences among groups in concentrations of oestradiol and prostaglandin F (PGF) in follicular fluid.