Age-related responses of serum testosterone to human chorionic gonadotropin in the stallion.

The response of serum testosterone to human chorionic gonadotropin (hCG, 10000 IU) given as a single intra-muscular injection was evaluated in middle age (n=3; 8, 9 and 10 years old) and old (n=2; 19 and 25 years old) stallions in nonbreeding (September) season. A sharp rise of testosterone after 2 hr was observed only in middle age stallions, but not in old stallions. However, 4872 hr after hCG administration, a clear and sharp increase was demonstrated in all stallions regardless of age. The different response of serum testosterone to hCG according to age should prompt a reevaluation for understanding of endocrine function of horse testis using this agent.