A serial histological study has been made of the ovaries of mice fed from the day of weaning on several different diets, the principal being a vitamin E deficient diet and one containing alpha‐tocopherol (“Ephynal” Roche) as the only vitamin E source. Control animals were kept on stock diet, lettuce being the main source of vitamin E. Animals were examined after periods ranging from 7 to 791 days of the particular diet.1. Animals on vitamin E rich diet, compared with controls, show ovaries with more primordial ova and primary follicles: follicular cells and interfollicular tissue of epithelial origin are more abundant, but degenerating follicles are numerous: there are fewer mature follicles and corpora lutea, both of which are enlarged. After prolonged vitamin E rich diet the ovary is small with degenerating follicles containing blood: neutral fat is present in the ovarian medulla and adipose tissue is extremely abundant in the mesovarium.2. Animals on vitamin E deficient diet, compared with controls, show ovaries with fewer primordial ova and less interfollicular tissue: follicles contain fewer follicular cells: corpora lutea are increased, but are smaller and of various ages: medullary lipids, not readily soluble in fat solvents, are prominent, yet there is complete absence of neutral fat in both ovary and mesovarium. As the vitamin E deficiency continues, the lipids invade the ovarian cortex. After prolonged vitamin E deficient diet the ovary, comparatively large, is hard and yellowish and contains only cells filled with lipids other than neutral fat.The ovarian changes summarised under 1 and 2 appear to be quantitative rather than qualitative; but it seems certain that vitamin E stimulates the development of all elements of germinal epithelium origin, and that it influences fat metabolism in the ovary and its neighbourhood so that neutral fat increases, while the formation of other lipids is inhibited.I wish to express my deep gratitude to Professor J. C. Brash, Professor of Anatomy, University of Edinburgh, under whose supervision the experiments have been performed, for his continuous readiness in advising, for providing accommodation and material for experiments, and also for revising the manuscript. I am deeply thankful to Professor T. Rogalski, Professor of Anatomy, Polish School of Medicine, Edinburgh, for guidance, advice, and suggestions received from him. I am very much indebted to Messrs Roche Products Ltd., Welwyn Garden City, England, by whom the synthetic vitamin E was generously provided. I owe thanks also to Mr. J. Borthwick, Anatomy Department, University of Edinburgh, for photomicrographs, and to all members of the staff of the Anatomy and Embryology Department of the Polish School of Medicine, who always willingly gave me technical assistance.