1. the interaction of unconjugated bilirubin with peritubular cell membranes of the rat kidney was studied by means of an isolated rat-kidney preparation applying the multiple-indicator-dilution technique.
2. Inulin was used as an extracellular marker and p-aminohippuric acid as a model of organic anion that interacts with the peritubular membrane.
3. A single renal artery injection of a mixture containing inulin and unconjugated bilirubin was followed by the appearance of the two compounds in the venous effluent. the unconjugated bilirubin curve was always under the curve of inulin and its mean transit time was always less than that for inulin.
4. the cumulative venous recovery of inulin was higher than that of unconjugated bilirubin.
5. When unconjugated bilirubin uptake was plotted against the injected dose of pigment the relationship suggested a saturation phenomenon.
6. the recovery of p-aminohippuric acid was significantly increased when unconjugated bilirubin was added.
7. the results provide evidence for the interaction of unconjugated bilirubin with the peritubular cell membranes of rat kidney.