NADPH-cytochrome P450 reductase (CPR) transfers electrons from NADPH to cytochrome P450 and also catalyzes the one-electron reduction of many drugs and foreign compounds. Various spectrophotometric assays have been performed to examine electron-accepting properties of CPR and its ability to reduce cytochrome $b_5$, cytochrome c, and ferricyanide. In this report, reduction of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) by CPR has been assessed as a method for monitoring CPR activity. The principle advantage of this substance is that the reduction of MTT can be assayed directly in the reaction medium by a continuous spectrophotometric method. The electrons released from NADPH by CPR were transferred to MTT. MTT reduction activity was then assessed spectrophotometrically by measuring the increase of $A_{610}$. MTT reduction followed classical Michaelis-Menten kinetics ($K_m;=;20;{mu}M$, $k_{cat};=;1,910;min^{-1}$). This method offers the advantages of a commercially available substrate and short analysis time by a simple measurement of enzymatic activity of CPR.