1. The fluorescence spectra of HQNO (2-n-heptyl-4-hydroxyquinoline-N-oxide) in water at pH 7.5 show an emission maximum at 480 nm and an excitation maximum at 355 nm. 2. The fluorescence is enhanced by binding to bovine serum albumin, and is completely quenched by binding to sub-mitochondrial particles of beef heart. 3. Binding experiments reveal specific binding of HQNO to sub-mitochondrial particles with a dissociation constant of 64 nM and, depending on the protein concentration, a considerable amount of aspecific binding. 4. The concentration of specific binding sites for HQNO is identical with that of antimycin-binding sites. Furthermore, the presence of antimycin prevents the binding of HQNO and antimycin releases HQNO from its binding site. 5. The binding of HQNO is not sensitive to the redox state of the respiratory-chain components. 6. Inhibition of electron transfer by HQNO is caused by binding to the specific binding site. 7. The relation between inhibition of NADH or succinate oxidation and saturation of the binding site is hyperbolic. 8. The increase in the reduction level of cytochrome b on addition of HQNO in the presence of succinate and oxygen, either in the presence or absence of cyanide, does not parallel the inhibition of overall electron transfer. 9. All data can be quantitatively described and analysed using the model for electron transfer proposed by Wikström and Berden in 1972 (Wikström, M.K.F. and Berden, J.A. (1972) Biochim. Biophys. Acta 283, 403-420).