Impact of progesterone (norethindrone) only oral contraceptive (MICRONOR) on mouse uterus was investigated by SEM. The three dimensional SEM photographs revealed that, flat wave like mucous folds of the uterus (control) become more thick and highly convoluted after 50 days of MICRONOR feeding. The uteri became highly secretory as indicated by increased thickness of nonciliated secretory cell population with their increased microvilli. Low doses of norethindrone in combination with circulating estrogen, instead of causing regression, maintained a steady and highly secretory state which may interfere implantation/gamete transport.