The effect produced by unconjugated bilirubin (UB) on p-aminohippuric acid (PAH) translocation from plasma to the urine, was analyzed by using an isolated rat kidney preparation applying the multiple indicator dilution technique. Arterial -to- urine mean transit times and fractional recoveries of creatinine and PAH in the venous effluent, were calculated before and after the incorporation of UB into the system. PAH influx into cells, its efflux from the cells to the capillaries, and its movement between lumen and tubular cells were also calculated. The results indicated that PAH influx into cells was impaired in the presence of UB. The phenomenom was shown to be dose-dependent. On the contrary, the efflux of PAH to the capillaries that was also impaired by UB, seemed to be unrelated to the pigment concentration within the system. Movement of PAH between lumen and tubular cells also appeared to be affected by UB. The results obtained suggested that UB interferes with the renal handling of PAH but the mechanism involved could not be clarified with the model used.