In guinea-pigs ovariectomized before implantation, where conceptuses continue to develop for some days, partial and finally complete haemorrhagic degeneration of the decidua takes place, usually terminating pregnancy by Day 16 or earlier. This can be prevented by exogenous progesterone (Deanesly, 1972). In pregnant rats, termination of pregnancy following ovariectomy is well known, but its immediate effect on the conceptuses does not appear to have been described. To compare their reactions with those in the guinea-pig, 6- to 9-day pregnant rats (vaginal plug, Day 0) were ovariectomized under ether anaesthesia and killed up to 2 days later, or after a longer interval if given progesterone. The 6- to 12-day conceptuses, fixed in Bouin's fluid and weighed from 70% alcohol, were compared with those from normal animals at corresponding ages. All were sectioned serially in the sagittal plane at 7 μm,