A single injection or two consecutive daily injections of oestradiol into intact ewes from Days 9 to 13 of the oestrous cycle induce premature luteal regression (Stormshak, Kelley & Hawk, 1969; Hawk & Bolt, 1970; Howland, Akbar & Stormshak, 1971). If the ewes are hysterectomized on Days 5 or 9 of the cycle, oestradiol administered between Days 9 and 13 does not cause regression of the CL, suggesting an involvement of the uterus in the effect of oestradiol on the CL (Stormshak et al., 1969; Bolt & Hawk, 1975). It has been proposed that the uterine luteolysin in the ewe is prostaglandin F-2α (McCracken et al., 1972), and oestradiol injection on Days 9 and 10 of the cycle results in an increased concentration of prostaglandin F-2α in uterine venous plasma 18 hr after the last injection (Ford et al., 1975).