Pharmacological Blockade of Ovulation in the Ewe

Investigation of hypothalamic areas responsible for release of gonadotrophins in sheep required a method for blocking the natural release of ovulating hormone(s). This report describes the results of preliminary observations on the effect of pentobarbitone sodium on ovulation in the ewe. In Merino ewes the onset of oestrus was determined by testing with vasectomized rams at intervals of no more than 2 hr. When ewes were found to be in oestrus they were either anaesthetized immediately with pentobarbitone sodium given i.v. and kept anaesthetized for 12, 24 or 48 hr., or anaesthetized for 12 or 20 hr. with pento-barbitone sodium commencing 4 hr. after the onset of oestrus or left conscious as controls. Four of the ewes anaesthetized at the beginning of oestrus were injected intramuscularly with 1000 i.u. human chorionic gonadotrophin (HCG) 6 hr. after the onset of oestrus. The ovaries of some ewes were examined at laparotomy 36