Two experiments designed to measure the influence of oestradiol on ovulation and cl maintenance in cycling ewes are reported. All oestradiol-treated ewes were given a single injection of oestradiol on the 4th day of their oestrous cycle ; in addition, some ewes were given continuous daily injections of progesterone or oestradiol. Ovarian condition was determined by gross observations in Exp. 1 and by gross and histological observations in Exp. 2. Oestradiol induced ovulation in 72 and 73% of the ewes in Exps. 1 and 2, respectively. Progesterone blocked the ovulatory response to oestradiol. Continued daily injections of oestradiol were effective in maintaining the cl and inhibiting follicular development. The oestrous behaviour exhibited by oestradiol-treated ewes was not associated with ovulation. Induction of ovulation by oestrogen is probably due to a release of lh. The luteotrophic nature of oestrogen in the ewe may be caused by an increase in prolactin secretion. lh as a luteotrophic factor is suggested.