The concentrations of oestradiol, androstenedione, progesterone and prostaglandin F2alpha (PGF2alpha) were measured in utero-ovarian autotransplants. The secretion of oestradiol was closely correlated with that of androstenedione (r = 0-67, P less than 0-001) indicating a common origin from the Graafian follicle. The concentration of these two steroids fluctuated at random throughout the luteal phase with the maximum secretion occurring about 2 days before the onset of oestrus. Functional regression of the corpus luteum, as indicated by a fall in the secretion of progesterone, began on day 12 or day 13, i.e. about 4 days before the onset of oestrus. In five of the six cycles the first significant rise in the secretion of PGF2alpha occurred on days 12-14 at the time of decline of progesterone secretion, although the release of PGF2alpha was maximal on the day before the onset of oestrus. There was very little release of PGF2alpha from the uterus before day 12. The temporal relationship of these events suggests that the uterus will only release PGF2alpha after it has been primed for 7-10 days with progesterone. The initiation of luteal regression is independent of secretion of oestradiol by the pre-ovulatory follicle which may, however, stimulate the further release of PGF2alpha responsible for irreversible structural luteolysis on the day of pro-oestrus.