1.Rotenone and other rotenoids have been shown to inhibit mitochondrial electron transport at a site on the oxygen-side of the NADH2 dehydrogenase (NADH2: (acceptor) oxidoreductase, EC 1.6.99.3) flavoprotein.2.The inhibition is characterized by a lag period of several minutes before the full effect of the inhibitor becomes apparent.3.It was confirmed that like antimycin A, the rotenoids become tightly bound to the mitochondrial particles. However, the relationship between the concentration of inhibitor and the degree of inhibition appears to have a hyperbolic form rather than the characteristic S-shape obtained with antimycin A.4.Sulphydryl reagents, such as N-ethylmaleimide and p-hydroxymercuri-benzoate, have a considerable potentiating effect on rotenoid inhibition.5.A study was made of the relationship between the chemical structure and inhibitory potency of a number of rotenoids and certain structural features essential for inhibition were established.6.The possibility that rotenoids and a number of other inhibitors and treatments are acting at a common site on the oxygen-side of the NADH2 dehydrogenase flavoprotein is discussed.