The role of the corpus luteum, the ovarian interstitial tissue and the ovarian cortex in the maintenance of the diapausing embryo was investigated by selective excision of the component tissues from female tammars undergoing seasonal quiescence. The response of the pituitary was assessed by measuring LH in peripheral plasma. No part of the ovary was necessary for the continued survival of the blastocyst in diapause for up to 4 months after ovariectomy, but at 8 months no blastocyst reactivated normally when the mother was treated with progesterone. Reactivation and the normal progress of pregnancy occurred in females deprived of the quiescent CL or the interstitial tissue only if a luteinized follicle or corpus luteum developed in the remaining ovary or a graft of ovarian cortical tissue. Plasma LH became elevated in bilaterally ovariectomized females, but not in unilaterally ovariectomized females or those carrying grafts of ovarian cortex.