The results of the present study indicate first that in the microsomal preparation, the components of the P-450 reduction system are heterogeneously distributed, comprising dissociable and nondissociable parts. Second, the P-450 reduction curve can be adequately described by a sum of two exponential functions, indicating two concurrent first-order reactions. Third, the two phases can be altered independently. The addition of the substrate increased the extent of the fast phase while it had little or no effect on that of the slow phase. Changes in the interaction of the dissociable and nondissociable components affected the extent of the slow phase while they were without effect on that of the fast phase. Experiments with different steroids indicated that the independence of the two phases is not due to functionally different P-450's and that the cytochrome reduced in both phases is essentially P-450C-21. The results are interpreted as follows: Transformation of P-450 from the low- to the high-spin state controls the total P-450 reduced. The rate and the biphasicity of the reduction are functions of the interaction of P-450 and the reductase.