Acyline is a novel GnRH antagonist found in animal studies to be a potent suppressor of circulating gonadotropin and testosterone (T) levels. Acyline inhibits basal and GnRH-stimulated serum testosterone concentrations in male dogs. Acyline prevented ovulation, but did not affect ovarian follicular development or gestational corpora lutea in the domestic cat. A single dose of the potent gonadotropin-releasing hormone antagonist acyline suppresses gonadotropins and testosterone for 2 weeks in healthy young men.