A mechanism for LHinduced regression of corpora lutea in rabbits

Summary. An explanation is proposed for the fact that luteinizing hormone (lh) injected into intact pseudopregnant rabbits acts in a luteolytic manner while in hypophysectomized rabbits it acts like a luteotrophin. Recent evidence shows that oestrogen is both luteotrophic and steroidogenic in rabbits. It is pointed out that injection of lh into intact rabbits causes ovulation of follicles which presumably serve as the endogenous source of oestrogen. In the absence of endogenous oestrogen, corpora lutea degenerate. In hypophysectomized rabbits, follicles do not ovulate in response to lh, and the latter is assumed to cause the unovulated follicles to secrete oestrogen which in turn acts in a luteotrophic manner on the corpora lutea. It has been noted by Stormshak & Casida (1964) that the injection of a single dose of lh (50 ^g) into rabbits causes the regression of corpora lutea (cl) of pseudopregnancy. At the same time lh has been found to be luteotrophic in hypophysectomized rabbits (Kilpatrick, Armstrong & Greep, 1964). It ha