Abstract:NADH-K3Fe (CN) 6 (FeCN) reductase activity was decreased to zero after 10 min or more of reaction. The activity was not recovered by the addition of substrates, NADH and FeCN, and plasmalemma together or separately. The probable reaction products, NAD+, Fe2+ and H+, were proved not to be as feed back inhibitors. The reaction medium containing plasmalemma, NADH and FeCN was centrifuged after incubation. It was found that the inhibitor was in the supernatant and the inhibitivity was lost after some six hous of storage.