Clinical experimentation was carried out to assess the effect of nonantiovulatory doses of norethindrone and depo provera on implantation in rats. Norethindrone in doses of 25 and 50 mcg and depo provera in doses of 5 and 10 mcg, administered from day 0-5 of pregnancy, prevented implantation. Lower doses of either drug had no effect on implantation. All doses of both drugs decreased the activities of several lysosomal enzymes as well as a proteolytic enzyme. Treatments with either norethindrone or depo provera after day 5 of pregnancy had no effect on the successful completion of the pregnancy. Depo provera was found to be 5 times more effective than norethindrone. The study results are tabulated.