Several 1- and 2-substituted 1H-imidazoles and 2-substituted oxazoles, oxazolines and pyrazines have been synthesized and tested as inhibitors of the cytochrome P 450 enzymes CYP2A6 and CYP2A13.  1-Substituted 1H-imidazoles bearing short chains (pentyl, hexyl or hexenyl) were found to be potent inhibitors of both enzymes, and showed IC50 values of about 2 microM.  The synthesis of several heterocyclic compds. (1- or 2-substituted 1H-imidazoles and 2-substituted oxazoles, oxazolines and pyrazines) has been achieved.  These compds. were tested as inhibitors of CYP2A6 and CYP2A13-two cytochrome P 450 enzymes present in the respiratory tract-with a view to preventing the formation of carcinogenic metabolites of nicotine and inhibiting the metab. of fragrances. 1-Substituted imidazoles bearing short alkyl chains displayed IC50 values of around 2 microM for both enzymes, together with high vapor pressures.