Placentation between 80 and 220 days of gestation was studied by measurement of fetal dimensions and gross, light and transmission and scanning electron microscopic examinations of the allantochorion and endometrium from 4 fertile mares with no, or very mild, endometrial histopathology and 6 subfertile Thoroughbred mares suffering varying degrees of age-related chronic degenerative endometritis (endometrosis). Spontaneously occurring twin gestation was observed in 3 animals. Several of the subfertile mares had endometrial cysts which showed 2 distinct features during pregnancy: those located near the lumenal surface of the endometrium showed no sign of microcotyledon development in both the endometrium and the opposed, flat and smooth allantochorion, whereas endometrial cysts located deeper in the stroma were covered by apparently normal glands and microcotyledons. Fetal dimensions were lower in the subfertile mares, but still within the reference range. Microcotyledon development appeared to be delayed and the chorionic macro- and microvilli looked shorter and blunter between 80 and 120 days in the 2 subfertile mares compared to the 2 fertile mares at the same gestational ages. At 120 days, the subfertile mare had significantly less microvilli per surface area than her fertile counterpart and her fetus weighed only half that from her counterpart, despite a similar crown-rump length. However, between 160 and 220 days the difference in microcotyledon development between the young fertile and old subfertile mares was much less marked, despite the continued existence of considerable numbers of distended 'gland nests' in the endometrium of the latter group.