The quinoline-based local anaesthetic cinchocaine (dibucaine) was found to be a mixed-type inhibitor of microsomal aminopyrine N-demethylase and 7-ethoxycoumarin O-deethylase activities from control and phenobarbitone-induced rat liver in-vitro. Cinchocaine also elicited a characteristic type I optical difference spectrum in oxidized liver microsomes (Ks = 24 microM; delta Amax = 3.4 X 10(-3) absorbance units (nmol cytochrome P450)-1) but did not appear to bind to the reduced form of the cytochrome. Additional studies indicated that cinchocaine competitively inhibited the type I spectral binding of substrate (aminopyrine) to ferric cytochrome P450. Studies of monooxygenase inhibition by cinchocaine over a relatively narrow pH range (6.5-8.5) indicated that, as might be expected, the un-ionized form of the drug is associated with inhibitory potency superior to that of the ionized form. Thus 40% inhibition of aminopyrine N-demethylase activity was observed with 100 microM cinchocaine at pH 8.0 and 8.5 (24% and 50% un-ionized drug, respectively), whereas only 16% inhibition was observed at pH 6.5 (1% un-ionized drug). These findings suggest that the inhibitory action of cinchocaine is mediated exclusively via an interaction with ferric cytochrome P450 and that the extent of ionization is a determinant of mixed function oxidase inhibition.