The reduction of ferric cytochrome P450 (P450) to ferrous is the first chemical step in almost all P450 reactions, and many characteristics of this step have been reported. Reduction kinetics of rabbit and human P450s were measured in a variety of systems. As reported earlier, P450 reduction is biphasic in microsomes and some purified P450 systems. However, this is not an inherent property of P450s, and some low- and high-spin iron P450s were reduced with single-exponential kinetics. Contrary to a generalized view, the presence of substrate is not necessary for rapid reduction of all P450s. Also, low-spin heme can be reduced as rapidly as high-spin in several P450s. P450s varied considerably in their reduction behavior, and even a single P450 showed remarkably different reduction kinetics when placed in various environments. P450 3A4 reduction was examined in liver microsomes, a reconstituted system, a fusion protein in which it was linked to NADPH-P450 reductase, and baculovirus and bacterial membranes in which P450 3A4 and NADPH-P450 reductase were coexpressed; the systems differed considerably in terms of the need for the substrate testosterone and cytochrome b5 (b5) for reduction and as to whether reduction was rate-limiting in the overall catalytic cycle. When b5 was included in reconstituted systems, it reduction kinetics were linked with those of some P450s. This behavior could be simulated in kinetic models in which electrons flowed from the ferrous P450.CO complex to oxidized b5. Overall, the kinetics of ferric P450 reduction cannot be generalized among different P450s in various systems, and concepts regarding influence of substrate, reaction sequence, and a rate-limiting step are not very universal.