The rate of nouphosphorylating electron transport (in the absence of ADP and inorganic phosphate) in weD-coupled (ATP/2e-= 0.9-1.1) maize mesophyD choroplasts Is not modulated by external pH (6.5-8.5), low levels of ADP or ATP, or energy transfer inhibitors, e.g. triphenyltin andHeg ions. In contrast nonphosphorylating electron flow in pea chloroplasts is sensitive to alterations In medium pH, and to the presence of adenine nucleotides and energy transfer inhdbitors in the assay medium Although ATP is without effect on the rate of basal electron transport in maize chloroplasts, steady-state proton uptake is stimulated 3-to 5-fold by low levels of ATP. These results suggest that differences may exist in the manner in which the coupling factor complex controls proton efflux from the intrathylakoid space in Cs and C4 mesophyll chloroplasts.A number of recent studies have examined the relationship between proton flux through the coupling factor complex (CFo-CF1) and electron flow in chloroplasts (2, 7-11, 24, 25, 27-29). In C3 mesophyll chloroplasts which have developed a large transmembrane proton concentration gradient, there appears to be a pathway for proton efflux through CFo-CF, that is not coupled to ATP synthesis (8,11,17,19,24,27,28 (8,17,24,27,28). Studies of the effects of energy transfer inhibitors, e.g. phlorizin, alkyl tins, Hg2+ ions, etc.on basal electron flow also suggest a role for CFo-CF, in the control of proton leakage from the intrathylakoid space via this pathway (8,11,27,28). Our aim was to compare the effects of adenine nucleotides and energy transfer inhibitors on the rate of nonphosphorylating electron flow and on proton fluxes in mesophyll chloroplasts isolated from both a C3 plant (pea) and a C4 plant (maize).