Respiratory chains of bacteria and mitochondria contain closely related forms of the proton-pumping NADH:ubiquinone oxidoreductase (complex I). In bacteria the complex has a molecular mass of approximately 530 kDa and consists of 14 different subunits. The homologues of these 14 subunits together with some 27 additional subunits make up the mitochondrial complex, adding up to a molecular mass of approximately 1 MDa. We calculated three-dimensional models at medium resolution of isolated and negatively stained complex I particles from Eschericha coli and Neurospora crassa by electron microscopy using the random conical tilt reconstruction technique. Both the bacterial and the mitochondrial complexes are L-shaped molecules with an intrinsic membrane arm extending into the lipid bilayer and a peripheral arm protruding from the membrane. It is discussed whether the consistent length of the arms of both complexes has an implication for their function. The additional protein mass of the mitochondrial complex is distributed along both arms, but especially around the junction between the two arms and around the membrane arm. It appears that the structural framework of procaryotic complex I is stabilized in eucaryotes by this additional mass. A discrete location of additional protein in the peripheral arm of the mitochondrial complex is interpreted as being the possible position of two subunits with a specialized role in the biosynthesis of a yet unknown cofactor of complex I.