Homologous 1-naphtoxyalcanthiols of the type 1-C10H7O(CH2)nSH (n = 2-7) are used for structural studies of the microsomal cytochrome P450 active centre. It was found that the strongest complex of thiol with P450 is formed for n = 3. Microsomal oxidation of P450 substrates aminopyrine and benz(a) pyrene is inhibited by the 1-naphtoxyalcanthiols studied. A non-monotonous dependence of pI50 on n was found, the compound with a chain length n = 3 appeared to be the most effective inhibitor. The interaction of this thiol (n = 3) with both the heme group of P450 and the hydrophobic substrate zone is supposed and the distance between these points was estimated. It is possible to employ this approach for structural studies on the active centers of different isoforms of P450.