The orientation of Rhodobacter sphaeroides reaction center complexes (RC complexes) in proteoliposomal membranes was investigated by a direct electrometric method. Conditions were found that allow monitoring of only that RC complex fraction that is oriented with its donor side to the inner part of the proteoliposome. It is shown that o-phenanthroline, an inhibitor of electron transfer between primary (QA) and secondary (QB) quinone acceptors, can also inhibit the photoinduced QA reduction. The efficiency of this inhibition depends on the concentration of added ubiquinone. It is assumed that the laser flash-induced o-phenanthroline inhibition of primary dipole (P-870+ · QA-) formation is of a competitive nature. © 1989 Plenum Publishing Corporation.