During gynaecological operations on 27 women aged 20 to 52 years, all visible follicles were punctured and 42 oocytes removed for investigation partly by phase-contrast microscope and partly by semi-thin slides. specimens of the walls of the follicles WERE REMOVED AND EXAMINED HIstologically. Furthermore, the activity of steroid-3 beta-ol dehydrogenase was demonstrated histochemically in 14 cases. Comparative investigations of the walls of the follicles and their oocytes gave the following results: 1. In non-ovulatory tertiary follicles (3 to 12 mm in diameter) with no, or very little luteinization of the theca interna and a granulosa of 3 to 10 cell layers, steroid-3 beta-ol dehydrogenase activity was only found in the theca interna. In 5 out of 23 follicles, the oocytes showed early or advanced signs of degeneration. Thus, atresia of the follicles probably occurs after degeneration of the oocytes. 2. 12 follicles (8 to 30 mm in diameter) showed distinct luteinization of the theca and steroid-3 beta-ol dehydrogenase activity in the theca interna, as well as in the granulosa. 4 of these follicles (all at least 20 mm in diameter) showed oocytes with typical pre-ovulatory changes. In 3 other cases with a similar morphological picture of the follicular wall specimens, the oocytes showed no preovulatory changes. The conclusion is drawn that the functional differentiation of the wall of the follicle induces the preparation of the oocyte for ovulation. On the other hand, 5 similar follicles already had degenerating oocytes. It remains doubtful whether such follicles can reach ovulation. 3. In follicles with early or advanced atresia, the oocytes were completely degenerated.