The appropriate model contains processes for influx into and efflux from both red blood cells and hepatocytes, as well as hepatocellular sequestration; however, in this case, part of the tracer emerges as labelled bicarbonate formed by the action of pyruvate dehydrogenase, whereas a presumably smaller part is incorporated into other metabolites by the action of pyruvate carboxylase. The model thus incorporates the formation of labelled bicarbonate as a metabolic product. The behavior of bicarbonate in the liver has previously been investigated using the multiple indicator dilution technique. Preliminary evaluation of the data showed that a substantial proportion of blood lactate is carried within red cells and therefore less readily available for hepatic uptake. The situation is similar to that for acetaminophen and will be treated in the same fashion. New experiments were therefore recently performed where lactate was injected in plasma alone (no red cells); the hematocrit in the bolus was re-established by simultaneously injecting blood where the hematocrit was increased by adding 51Cr-labelled red cells (non-pre-equilibrated case), as previously done with thiourea.