Disclosed herein is a porcine intra-vaginal device of a shape and size adapted to be positionable in the vaginal tract across the hymenal ring of a target animal (e.g., a gilt) to extend to both sides of the hymenal ring of the animal. The device once inserted delivers progesterone from a progesterone impreganated matrix on either side of the hymenal ring, the progesterone releasing surface being at least 150 cm.sup.2 in total area. Variable geometry means (preferably for vestibular engagement) ensure retention of the devices (e.g., for at least 7 to 14 days) where, in the preferred device, the progesterone load of from 1.9 to 2.5 g within 1.2 mm of the release surface can, by maintaining a progesterone blood plasma level (equating to a progestrone blood plasma level in excess of 4 ng/mL measured in an ovariectomised animal), ensure or prompt the onset of oestrus within 3 to 5 days after device removal.