In connection with a previous study, new phenylindoles bearing a 2- or 3-thienyl group were synthesized and tested as specific inhibitors of mitochondrial NADH dehydrogenase. The position of the phenyl ring and the geometrical configuration play an important role in the activity and specificity of these derivatives. In order to study the mechanism of action of these thienylvinylindoles, their activity was compared with that of known inhibitors in a new test employing exogenous quinones.