where subscripts e and c refer to extracellular and cytosolic spaces, respectively, several lines of evidence support the assumption that near-equilibrium conditions exist for processes 1 through 3 during the normoxic steady state of rat liver [l-31.Since 1965, in studies with the isolated hemoglobin free perfused organ [4,5], extensive use was made of this system in the direction from right to left, using the ratio interest was focussed on the other direction in order to obtain detailed information on the NADH system of liver oytosol which was previously not accessible. Extracellular [lactate]/[pyruvate] was used as experimental variable, and dependent changes in cytosolic NADH were observed by a variety of methods, in particular by continuous measurement of fluorescence and absorbance of perfused liver a t pyridine-nucleotide-specific wavelengths.By means of mathematical extrapolation of titration results in a double reciprocal plot derived on the basis of the mass action law, the maximal