The paired-tracer method has been used extensively for determining cell uptake of numerous substances, although the method of calculating uptake has no published theoretical support. We have investigated the effect of capillary permeability of the tracers on v, the uptake rate calculated directly from the ratio of tracer venous concentrations. For a simple mathematical model of plasma-tissue movement of lactate and an analytic expression for v, it has been shown that values of v calculated in the first moments after tracer injection depend almost entirely on the differences in tracer permeability-surface area product (PS). The model indicates v would never give the correct value of cell uptake. It is also shown that PS differences alone can explain the published values for lactate uptake obtained from v in skeletal muscle of the rat and dog.