A study is presented of the action of triphenyltin on the kinetics of the anaerobic relaxation of the proton gradient set up by respiration in various type of 'inside-out' inner membrane vesicles obtained by exposure of beef-heart mitochondria to ultrasonic energy.Triphenyltin is shown to act as a powerful inhibitor of the proton conductivity of the H+-ATPase. The inhibition persists after removal of the ATPase protein inhibitor, F 1 and the oligomycin-sensitivity conferral protein (OSCP) from the particles. The inhibitory effect of triphenyltin is exerted, as in the case of oligomycin and N,N'-dicyclohexylcarbodiimide, on the Fo moiety of the ATPase complex.Comparison of the characteristics of the effect of triphenyltin on proton translocation in chloride and nitrate media shows that the inhibition of passive proton conductivity studied here is unrelated to the hydroxide/ anion exchange induced by the organotin.Lack of additivity of the inhibition of H+ conduction by triphenyltin with that exerted by oligomycin and N,N'-dicyclohexylcarbodiimide and the kinetic pattern of the effect of triphenyltin show that the mechanism of action of the organotin is different from that of the other two inhibitors.The relevance of the results obtained with respect to the subunit location and chemical nature of the reaction site of triphenyltin in the H+-ATPase complex is discussed.The membrane sector, Fo, of the H+-ATPase of coupling membranes functions as proton translocator [l -71. In the intact H+-ATPase complex proton translocation by FO is compulsorily coupled to the hydrodehydration reaction catalyzed by the peripheral, F1 moiety of the complex [1,2]. Displacement or removal of F1 from the complex, as occurs in 'insideout' submitochondrial particles, unmasks the proton conductivity of FO [5,6,8 -101. Thus anaerobic relaxation of the electrochemical proton gradient, dpH', set up by respiration in these vesicles, takes place through the proton conducting pathway of FO [5,6,8,9]. FO preparations from different sources consist of a variable number of protein subunits [I 11. The use of DCCD, a specific reagent for glutamic and aspartic residues [I21 which inhibits proton conduction by Fo [1,2,6], has shown that a 7000-8000 proteolipid subunit of Fo, bearing a glutamic [13] or aspartic residue [I41 specifically attacked by DCCD [15], is directly involved in proton translocation [l, 21. Modification with phenylglyoxal of arginine residues and of tyrosine with tetranitromethane results in depression of the proton conductivity of FO of the thermophilic bacteria PS3 [16,17] and mitochondria [S, 91.Organotin compounds, besides their activity in inducing hydroxide/anion exchange-diffusion [18,19], inhibit the ATPase of mitochondria [19-211 and chloroplasts [22,23]. There are reports that this is probably due, as with oligomycin and DCCD [1,2,6], to depression of H+-conduction by FO [23,24]. The inhibition of the H+-ATPase of mitochondria by triAhhrcviurions. DCCD, N,N'-dicyclohexylcarbodiimie; ESMP, submitochondrial particles prepare...