NADPH-cytochrome P-450 reductase and cytochrome P-450, both purified from liver microsomes of phenobarbital-treated rabbits, were incorporated into dimyristoylphosphatidylcholine vesicles. The reduction of cytochrome P-450 by NADPH in the reconstituted vesicles proceeded in a biphasic fashion, and 70-80% of the absorbance change was associated with the fast phase. The Arrhenius plot of the apparent first-order rate constant of the fast-phase reduction showed a marked discontinuity around the phase transition temperature of the synthetic phospholipid; an almost 10-fold change in rate constant was associated with this discontinuity. It was, therefore, suggested that the reduction of cytochrome P-450 by reductase in this system was a diffusion-limited reaction controlled by the viscosity of the phospholipid membrane. The Arrhenius plot of overall drug monooxygenase activity catalyzed by the reconstituted vesicles showed a break but in a different way from that observed for the reduction of cytochrome P-450. This break was accompanied only by a change of the slope of the plot but not by a change in reaction rate. This difference in the two Arrhenius plots was attributed to that in the rate-limiting step of the two reactions. NADPH-cytochrome c reductase activity of the reconstituted vesicles, an activity catalyzed by the reductase alone, and cumene hydroperoxide dependent N-methylaniline demethylation activity catalyzed by cytochrome P-450 alone did not show any break in the Arrhenius plots.