The proton-pumping NADH:ubiquinone oxidoreductase, also called complex I, is the first of the respiratory complexes providing the proton motive force which is essential for the synthesis of ATP. Closely related forms of this complex exist in the mitochondria of eucaryotes and in the plasma membranes of purple bacteria. The minimal structural framework common to the mitochondrial and the bacterial complex is composed of 14 polypeptides with 1 FMN and 6-8 iron-sulfur clusters as prosthetic groups. The mitochondrial complex contains many accessory subunits for which no homologous counterparts exist in the bacterial complex. Genes for 11 of the 14 minimal subunits are also found in the plastidial DNA of plants and in the genome of cyanobacteria. However, genes encoding the 3 subunits of the NADH dehydrogenase part of complex I are apparently missing in these species. The possibility is discussed that chloroplasts and cyanobacteria contain a complex I equipped with a different electron input device. This complex may work as a NAD(P)H: or a ferredoxin:plastoquinone oxidoreductase participating in cyclic electron transport during photosynthesis.