PIP: The biliary excretion and intestinal metabolism (including intestinal mucosa and small intestine and bowel) of progesterone and estrogens in humans are reviewed here along with presentation of experimental results from other mammalian systems. In general, the biliary excretion of estrogens and the enterohepatic circulation of estrogen metabolites are more extensive than for progesterone. These processes may be of greater physiological importance because of the possible reformation of biologically active estrogens in the intestine, which occurs 2 ways: 1) by hydrolysis of biliary estrogen conjugates and absorption of the unconjugated estrogens which may partly reach general circulation, and 2) by production of biologically active estrogens from neutral steroids or less active estrogens in the intestinal tract, followed by absorption. Here, the kidneys also play a significant role in estrogen metabolism and conjugation. The quantitative contribution of liver, kidneys, and intestine to estrogen metabolism cannot be accurately assessed at present, but the liver and intestines probably play the most significant role, followed by the kidneys. Progesterone (from orally administered doses) and its metabolites are extensively metabolized in the intestine to compounds with less progestational activity; hence, synthetic progestins may alter the intestinal flora which in turn may influence the plasma levels of these compounds. On these subjects more research in indicated.