Cytochrome P-450 LM2 reduction was measured at a series of NADPH concentrations in the absence of substrate and in the presence of 1 mM benzphetamine. In the absence of substrate reduction could be described as a biphasic process with 55% of the reaction occurring in the first phase (at 20 microM NADPH). When benzphetamine was present, the fraction of the reaction occurring in the first phase was increased to 91%. When examined either in the absence or presence of benzphetamine, the rate constant and fraction of LM2 reduced in the fast phase were decreased as the NADPH concentration was decreased. In each case the fraction of LM2 reduced in the second phase was not substantially altered over the NADPH concentrations examined. To explain the effect of NADPH concentration on the initial rate of LM2 reduction, the effect of NADPH on the reduction of NADPH-cytochrome P-450 reductase was examined. Due to the presence of two flavins within each reductase molecule, there would be nine possible oxidation-reduction states of the reductase which may be present at a given NADPH concentration. Based on the redox potentials for the flavin half-reactions and for NADPH oxidation, the relative concentrations of each of the reductase subspecies could be determined. Rate constants were assigned for the reduction of LM2 by the various reductase subspecies, and the theoretical initial rates of LM2 reduction at various NADPH concentrations were compared with values obtained experimentally. The experimental data are consistent with a model where, under the conditions of this assay, the fully reduced reductase is the form primarily responsible for the reduction of LM2.