4(1H)-quinolones (2-alkyl- (1), 2-alkyl-3-methyl- (2), 2-methyl-3-alkyl- (3), 1-hydroxy-2-methyl-3-alkyl- (4) and 1-hydroxy-2-alkyl- (5)) with n-alkyl side chains varying from C5 to C17 have been synthesized and tested for biological activity in mitochondrial complexes. Whereas all quinolones were efficient inhibitors of electron transport in the cytochrome b/c1-complex from either beef heart or Rhodospirillum rubrum, in complex I from beef heart quinolones 1 and 2 only were highly active. In a Quantitative Structure-Activity Relationship (QSAR) inhibitory activity in the cytochrome b/c1-complexes could be correlated to the physicochemical parameters lipophilicity π and/or to STERIMOL L. Maximal inhibitory potency was achieved at a carbon chain length of 12–14 Å. Oxidant-induced reduction of cytochrome b established that some quinolones are inhibitors of the Qp rather than the Qn site.