Hypophysial blockade, achieved by the injection of large doses of progesterone into pigs, and begun one or 2 days before ovulation, on the day of ovulation, or one day after ovulation, could not prevent the formation and maintenance of corpora lutea for 14 days—the normal duration of the luteal phase in pigs. In spite of continuous hypophysial blockade for 7–10 days, the formed corpora lutea contained nearnormal amounts of progesterone at autopsy. We conclude that, in animals in which ovulation occurs, corpora lutea need no sustained hypophysial impetus (beyond that obtained at ovulation) for normal structure and function. (Endocrinology74: 9, 1964)