The oral administration of 5 mg. quinestrol (3-cyclopentyl ether of ethinyl oestradiol) has been shown to inhibit ovulation in normal women (Skerlavay & Epstein, 1967); presumably the quinestrol blocks the release of gonadotrophins. In parabiotic rats, Giannina, Steinetz & Meli (1967) used daily administration of oral quinestrol to block hypersecretion in the castrated male.
In the present study, hemicastrated adult female rats were used to test the prolonged pituitary blocking effect of a single oral dose of quinestrol. At the time of the unilateral ovariectomy, the rats recieved 1 mg. quinestrol by gavage. Animals were killed at approximately 2-day intervals after operation. At autopsy the remaining ovary (the left) was removed and weighed. Hemicastration produces a compensatory hypertrophy of the remaining ovary which may be prevented by oestrogen administration (Peterson, Edgren & Jones, 1964). If quinestrol is long-acting when given orally and suppresses pituitary gonadotrophic function it should decrease this