The reactions of NADPH- or dithionite-dependent reduction of cytochrome P-450 were studied using a stopped flow technique. It was found that the kinetic curves for both reactions may be fitted by a sum of the two exponents. The arrhenius plots for the fast phase rate constants are linear for both reactions. On the contrary, the breaks on the corresponding plots for the slow phase rate constants are observed at 22 and 33 degrees C for cytochrome P-450 reduction by dithionite and at 31 degrees C for NADPH-dependent reduction of cytochrome P-450. The coincidence of the values of the rate constants and activation energy (56 +/- 5 kJ/mol) for the fast phase of NADPH-dependent reduction of cytochrome P-450 with values of catalytic constants and activation energy for demethylation of tertiary amines suggests that the first electron transfer process from NADPH-cytochrome P-450 reductase to cytochrome P-450 may be the rate-limiting step. A diverse character of the kinetic parameters for the two cytochrome P-450 reduction reactions is indicative of different nature of biphasity of these processes.