Propylthiouracil inhibited the activity of NADH-cytochrome b5 reductase of rat liver microsomes using potassium ferricyanide as electron acceptor. On the other hand, NADPH-cytochrome P-450 reductase activity was not affected by the compound. NADH-supported reduction of cytochrome b5 was also inhibited by propylthiouracil in the reconstituted system consisting of cytochrome b5 and partially purified NADH-cytochrome b5 reductase.