Treatment with human chorionic gonadotropin (hCG) for ovulation induction is associated with an immediate 17β-estradiol decrease and a more rapid LH increase in mares

The effect of treatment with an ovulation-inducing dose of human chorionic gonadotropin (hCG) on 17β-estradiol (estradiol) and LH concentrations was studied in mares. In Experiment 1, treatment with hCG resulted in ovulation in approximately 48 h. The LH surge centralized to ovulation and the preovulatory decline in estradiol were not different between hCG-treated (n = 15) and control (n = 13) groups. In Experiment 2, concentrations of hCG decreased 92% between 1 and 48 h after hCG treatment, estradiol decreased (P < 0.003) within 6 h, and LH increased at a greater (P < 0.02) rate after 12 h (n = 14). Results indicated: (1) hCG induced a decrease in estradiol and a subsequent greater rate of increase in LH and (2) when centralized to ovulation, the endogenous LH surge and the estradiol decrease were similar between hCG-treated and control groups.