The photosynthetic election transport activities in beet spinách thylakoids were studied using ruthenium chloride as an electron acceptor, Like potassium ferricyanide, RUCI3 supported the non-cyclic electron flow with net evolution of oxygen. The rate of oxygen evolution was at its maximum with 0.5 mM RUCI3 at pH of 8.0 and the election flow coupled to translocation of protons into the thylakoid vesicles. Ruthenium chloride-supported oxygen evolution was inhibited by specific photosynthetic electron tiansport inhibitors like diuron, dibromothymoquinone, potassium cyanide, and mercuric chloride Unlike ferricyanide, the RuCl3-supported oxygen evolution was totally inliibited by potassium cyanide and mercuric chloride at both pH 8.0 and 6.5. Since potassium cyanide and mercuric chloride mostly interrupt the electron flow at plastocyanin level, RUCI3 probably accepts electrons mostly from photosystem 1 or its near vicinity. Besides electron acceptance, RUCI3 suppresses the photophosphoiylation activity in a manner similar to energy transfer inhibitors.