Metaplastic charges due to vitamin A-deficiency and overstimulation with estrogen were studied in the uterus of the rat. The investigation was concerned with the following problems: (1) the role of the ovaries in producing uterine metaplasia in vitamin A-deficient rats; (2) whether uterine metaplasia which is induced by estrogen is a gradual process or a rapid one, which takes place a short time after the beginning of treatment; (3) whether the age of the animal at the start of estrogen treatment has any effect on the production of metaplasia; and (4) a cytologic comparison of the metaplastic processes resulting from vitamin A-deficiency with those resulting from overstimulation with estrogen.