The isoflavonoids in soy, genistein and daidzein, have been proposed to contribute an important part of the anti-cancer effect of soy. Although there have been many interesting studies on the effects of isoflavones on biochemical targets in tissue culture experiments, in most cases the concentrations used by investigators have exceeded 10 microM. However, based on simple pharmacokinetic calculations involving daily intake of isoflavones, absorption from the gut, distribution to peripheral tissues, and excretion, it is unlikely that blood isoflavone concentrations even in high soy consumers could be greater than 1-5 microM. Experiments designed to evaluate these pharmacological principles were carried out in anesthetized rats with indwelling biliary catheters and in human volunteers consuming soy beverages. The data from these experiments indicate that genistein is efficiently absorbed from the gut, taken up by the liver and excreted in the bile as its 7-O-beta-glucuronide. Re-infused genistein 7-O-beta-glucuronide was also well absorbed from the gut, although this occurred in the distal small intestine. In human subjects fed a soy beverage for a period of two weeks, plasma levels of genistein and daidzein, determined by HPLC-mass spectrometry, ranged from 0.55-0.86 microM, mostly as glucuronide and sulfate conjugates. In summary, genistein is well absorbed from the small intestine and undergoes an enterohepatic circulation. Although the plasma genistein levels achievable with soy food feeding are unlikely to be sufficient to inhibit the growth of mature, established breast cancer cells by chemotherapeutic-like mechanisms, these levels are sufficient to regulate the proliferation of epithelial cells in the breast and thereby may cause a chemopreventive effect.