Methods are described for the determination of bioavailability of lipo‐ and hydro‐soluble compounds in the rat. These procedures involve catheterization of the portal vein and/or intestinal lymphatics to study absorption in unanesthetized animals. Catheterization of the common bile duct prevents recycling of materials through the entero‐porto‐hepato‐biliary circulation (EPHBC). Steady‐state conditions are ensured by constant infusion of bile or a solution of bile acids into the stomach or duodenum. The techniques and physiological considerations discussed in detail here have resulted in new proposed animal preparations, of value in the accurate determination of the bioavailability of labeled residues ingested by the second species. Since intake must be adequate to permit meaningful conclusions, the concentration of radioactivity in the harvested homogenized tissue is increased by lyophilization. The lyophilized material is compressed into pellets of adequate size. The animals, kept in restraining cages to prevent coprophagy, are allowed to eat the labeled residue spontaneously, if necessary, for 48 and even 12 hr. Bile, urine, and feces are collected for a sufficient length of time to allow quantitative excretion of the labeled residue, if no absorption takes place. Co/lection of these excreta must be complete. It is essential to account for most—if not all—of the radioactivity administered. Data so obtained allow an accurate balance between intake and excretion of the labeled residue. The presence of radioactivity in the intestinal wall, carcass, liver, and urine is indicative of absorption. The appearnace of radioactivity in the bile also indicates absorption, in addition to suggesting that the compound(s) may undergo EPHBC. The extent of recovery of the administered radioactivity in the luminal contents and feces indicates the extent to which the labeled residue is not bioavailable. Evaluation of data obtained with these new animal models should permit corresponding upward adjustments of the minimum levels of residues allowable in the tissue of animals intended for human consumption.