The effects of 2-n-heptyl-4-hydroxyquinoline-N-oxide on electron transport in thylakoids and oxygen-evolving photosystem II particles has been examined. Kinetic fluorescence studies reveal that the site of inhibition for alkyl derivatives of hydroxyquinoline-N-oxide (I50 approximately equal to 2 microM) is located between Q and plastoquinone. Studies with thylakoids isolated from atrazine-resistant pigweed plants indicate that the modification in the Q/B membrane complex that confers increased resistance to inhibition by atrazine also results in decreased sensitivity to inhibition by 2-n-heptyl-4-hydroxyquinoline-N-oxide (resistant/ sensitive ratio = 11). From the results of tetramethylphenylenediamine by-pass experiments, determinations of inhibitor sensitivity in trypsin-treated thylakoids and competitive displacement experiments made with [14C]metribuzin in thylakoids and photosystem II particles, it is suggested that 2-n-heptyl-4-hydroxyquinoline-N-oxide binds in a region of the Q/B complex that is distinct from the 3-(3,4-dichloro)-1,1-dimethyl urea and atrazine binding sites.