SINCE the original observation by Loeb (11) that hysterectomy of the guinea pig during the luteal phase caused maintenance of the corpora lutea, a valid physiological mechanism has been sought to explain this phenomenon, which has been recorded in a number of other species (see review 1). Pharris and Wyngarden (18) proposed that since prostaglandin F2α (PGF2α) is an abundant uterine prostaglandin (19) and since it has a pronounced venoconstricting effect (5), it is potentially a substance which might control luteal regression. They reported that PGF2α administered subcutaneously to rats at 1 mg/kg/day caused a dramatic shortening of pseudopregnancy (18). A similar effect was observed in the pseudopregnant hysterectomized hamster (8) and in the hysterectomized guinea pig (3). This paper reports the effect on ovarian blood flow and progesterone secretion rate of infusing PGF2α into the sheep ovary autotransplanted with vascular anastomoses to the vessels of the neck (7).