Summary. Ovarian function and early development of the embryo was investigated histologically in vampire bats removed at timed intervals after mating from a laboratory colony. Both ovaries were equally functional, but a single ovum only was released at each oestrus. The preovulatory follicle or new CL was in the opposite ovary from the old CL in most bats possessing old CL. Large preovulatory follicles were observed as late as Day 5 (Day 1 = first day of sperm-positive vaginal smears). Early cleavage stages were recorded on Days 3 to 8. Advanced morulae and early blastocysts sometimes lacking zonae pellucidae were observed in the oviducts on Days 15 and 16.