Acetaminophen uptake and conversion in the perfused rat liver to acetaminophen sulfate was studied with the multiple indicator dilution technique (MID). Because acetaminophen is avidly bound to red blood cells and not albumin, a pre-equilibrated MID dose containing the noneliminated references (51Cr-labeled red blood cells [RBC, a vascular reference], [58Co]EDTA [a small molecular weight interstitial reference that does not enter cells], and D2O [a cellular reference]) and [<3>H]-acetaminophen was introduced into the portal vein of the single-pass perfused rat liver (1 mg/L acetaminophen) under varying conditions of hematocrit, with observation of timed outflow profiles in the hepatic venous blood. The [<3>H] acetaminophen curve exhibited an early high peak, paralleling that for red cells and varying with hematocrit, followed by a prolonged decline, with the late appearance of acetaminophen sulfate product; the early peak disappeared when red cells were absent from the dose and perfusate. Analysis demonstrated a slow release of acetaminophen from the red blood cells and rapid liver cell entry, so that red cell binding was displayed as a red cell carriage effect that reduced the rate of liver cell entry and hence of sulfation of [<3>H]-acetaminophen. The liver cells exhibited a concomitant very low permeability to product acetaminophen sulfate, leading to protracted product outflow curves. An inferred slow efflux-mediated storage phenomenon for product was found to evolve as a result. (HEPATOLOGY 1995; 22:267–282.)