The site of action of 'chlormadinone acetate' (6-chloro-delta 6-dehydro-17 alpha-acetoxyprogesterone) in blocking ovulation in the mated rabbit.

1. In a study of the site of action of a steroid in current use for contraceptive purposes (6-chloro-Delta(6)-dehydro-17alpha-acetoxyprogesterone; ;chlormadinone acetate'), the ovarian responses [secretion rate of 20alpha-hydroxypregn-4-en-3-one (20 alpha-OH), and the occurrence of ovulation] were observed in control oestrous rabbits and in rabbits following mating, injection of luteinizing hormone, and after electrical stimulation of the median eminence.2. Chlormadinone acetate pretreatment (0.5 mg I.V.) did not lower significantly the secretion of 20 alpha-OH in otherwise untreated oestrous rabbits.3. Chlormadinone acetate, given 24 hr before mating, prevented the rise in 20alpha-OH that would otherwise have occurred, and also blocked the ovulation response following mating.4. Chlormadinone acetate pretreatment did not prevent the ovarian responses to administration of luteinizing hormone.5. Chlormadinone acetate pretreatment did not prevent the ovarian responses to electrical stimulation of the median eminence of the tuber cinereum of the hypothalamus.6. The conclusion is drawn that the chlormadinone acetate block of the ovarian responses following mating is at a site in the central nervous system located above the median eminence.