The action of chlormadinone acetate (6-chloro-delta6-dehydro-17alpha-acetoxyprogesterone) upon experimentally induced ovulation in the rabbit.

1. The ovulation response to electrical stimulation of the median eminence, or to the intrapituitary infusion of median eminence extract, was observed in control rabbits and animals pre-treated with chlormadinone acetate.2. Chlormadinone acetate pre-treatment did not significantly reduce the proportion of rabbits ovulating in response to electrical stimulation. However, there was a significant reduction in the average number of ruptured follicles when compared with the animals in the control group.3. Chlormadinone acetate pre-treatment significantly reduced the proportion of animals ovulating in response to the intrapituitary infusion of median eminence extract as compared with the control series. However, there was no significant reduction in the average number of ruptured follicles (in those rabbits that ovulated) when compared with the control groups.4. Chlormadinone acetate pre-treatment produced no significant effect upon either the proportion of animals ovulating, or the average number of ruptured follicles, following I.V. injection of 50 mug LH.5. The conclusion drawn is that chlormadinone acetate blocks copulattion-induced ovulation in the rabbit by action at a site in the central nervous system located above the median eminence. The possible effects of chlormadinone acetate upon the secretion of FSH, the pituitary sensitivity to releasing factors and the ovarian sensitivity to LH are discussed.