The effects of several respiration inhibitors on photosystem 11 (PS 11) were investigated. Among the agents tested, piericidin A and thenoyltrifluoroacetone (TTFA) inhibited the photosynthetic electron transport of spinach as measured from chlorophyll (Chl) fluorescence parameters (Fm'-F)/Fm' and Fv/Fm. Using specific donors and acceptors of electrons, we identified the sites of inhibition in and around the PS II complex; the site of inhibition by TTFA was between Q(A), primary quinone acceptor in PS 11, and Q(B), secondary quinone acceptor, in the acceptor side of P680, the reaction center Chl of PS 11, while inhibition by piericidin A of the acceptor side was downstream Of QB, out of the PS 11 complex. Both agents also inhibited the donor side of P680, probably between tyrosine-161 of the reaction center protein of PS 11 and P680.