Department of Zoology, University of Tennessee, Knoxville, Tennessee 37916, U.S.A. (Received 8th July 1974) The North American opossum, Didelphis marsupialis virginiana, is a polyovular, polyoestrous marsupial. The gestation period of 13 days is the same length as the luteal phase of the cycle (Hartman, 1923, 1925); the next pro-oestrous phase and ovulation are suppressed by the presence of sucking young in the pouch. During the breeding season, removal or loss of the young results in resumption of follicular growth and oestrous behaviour is evident 3 to 7 days later (Hartman, 1923; Renfree, 1974). After ovulation, the two uteri increase greatly in size and turgidity, due to an accumulation of fluid in the hypertrophied mucosa and the development of a rich supply of capillaries and venules (Harman, 1923). Ciliated surface cells are involved in the deposition of much ground substance in the stroma, and the secretory cells of the glands have