Study on the effect of pentachlorophenol on the succinate oxidase activity of submitochondrial particles and on the reduction level of cytochromes b revealed that the Ki value for PCP is equal to 2-4 microM. The succinate-DCPIP-reductase activity is noncompetitively inhibited with PCP (by 75-85%) (Ki = 3.6 microM). In the case of the succinate-PMS-reductase activity PCP at micromolar concentrations decreases the value of V only by 40% (C50 = 2 microM) with a simultaneous increase of the Km value for PMS. The identity of Ki values for PCP under these conditions suggests that the effect of PCP is due to the inhibitor interaction with the same component of the succinate dehydrogenase complex. The type of action of PCP on the succinate-acceptor-reductase activities indicates that the inhibiting effect of PCP on succinate oxidations is similar to that exerted by traditional inhibitors of succinate dehydrogenase--tenoyltrifluoroacetone and carboxins. Since PCP inhibits succinate dehydrogenase at low concentrations, it seems likely that the biological (pesticidal) effect of PCP is provided for not only by its uncoupling action but also by the inhibition of succinate oxidation in the respiratory chain.