Clinical experimentation was carried out to assess the antiestrogenic effects of norethindrone and depo provera administered at a dose schedule which did not inhibit ovulation. The laboratory procedures employed are explained and results tabulated. Norethindrone at doses of 10, 25, and 50 ug/rat daily for 10 days and depo-provera at doses of 1, 5, 10 ug/rat daily for 10 days did exhibit antiestrogenic effects. Both substances caused a reduction in uterine water nucleic acid, and protein content. The results were dose-dependent. Both were more potent when administered at the estrus than at the proestrus phase. Discontinuation of treatment for 10 days normalized the changes with smaller doses taking less time to reach normalization than larger doses. The effect of norethindrone persisted in the proestrus phases while the effect of depo-provera persisted in the estrus phase. Depo-provera was found to be 5 times more potent than norethindrone.