Arachis oil instilled into the uterus of sensitized mice was localized mesometrially or, more commonly, antimesometrially, suggesting that the uterus is polarized in its capacity to respond since implantation chambers only form antimesometrially. Epithelial breakdown occurred only within the `implantation chamber', but cell death took place more rapidly than in normal pregnancy and was complete at only 5 to 9 hr after the Pontamine Blue reaction. Between 19 and 43 hr after oil instillation, the antimesometrial epithelial cells lost contact with each other and initially were characterized by distended rough endoplasmic reticulum and Golgi body, lipid droplet accumulation and ribosome segregation. At a later stage of deterioration, epithelial cell contents were scattered into the uterine lumen where polymorphonuclear leucocytes and monocytes were also found, probably involved in ingesting the cellular debris. There was no evidence of increased size of lysosomal dense bodies or of the formation of autophagosomes in dying epithelial cells; suggesting that the mechanism of epithelial death in the oil-induced reaction is not identical to that of normal pregnancy.