Histological changes were studied in the ovary and uterus of rats receiving different doses of Depo-Provera (DMPA) and Noristerat (NET-EN) for varying duration. The effect of DMPA on ovarian and uterine tissues was strongly progestational. The whole morphological alteration in the ovary after DMPA therapy appeared to be the atresia of the follicular apparatus with degeneration of the growing follicles. Uterine histology reflected that endometrial tissues gradually became inactive and with prolonged treatment at high doses, atrophy of the endometrium was noted. In NET-EN-treated rats, absence of mature follicles and recent corpora lutea reflected the blockade of ovulation. There was no extreme atrophy of the ovary or endometrium as found with DMPA treatment. With higher doses of NET-EN, endometrial growth was arrested. © 1988.