S U M M A R YThe respiratory electron transport system involved in NADPH (2.0 nmoles/ min./mg.) and NADH (1.0 nmoles/min./mg.) oxidation in the dark that was operative in extracts of light-grown Anabaena variabilis has been examined.NADPH oxidation was inhibited 50 % by cyanide (2 x I O -~ M), rotenone (IO-* M); antimycin A, amytal and azide were markedly less inhibitory. NADPH : ferrocytochrome c oxidoreductase, NADPH : rnenadione oxidoreductase, ferrocytochrome c : oxygen oxidoreductase and succinic dehydrogenase were detected. A phosphorylation (0.4 nmoles/min./mg.), associated with NADPH oxidation, was measured, NADPH could be replaced by NADP and isocitrate. This phosphorylation was absolutely dependent upon oxygen and was inhibited 25 yo by carbonyl cyanide p-trifluoromethoxyphenylhydrazone (5 p~) .