Abstract When the local anaesthetic drug lidocaine is added to liver microsomes biphasic type I spectral change titration curves can be observed. A high-affinity and a low-affinity phase is observed. In the present study we have found that microsomes from female rats have a dominant high-affinity phase, which can hardly be observed within microsomes from female guinea pigs. Male rats showed an intermediate phase. On incubation of lidocaine at concentrations of 1 γM or less with female rat liver microsomes a larger fraction of the drug was aromatically hydroxylated than deethylated. The opposite was true for guinea pig liver microsomes, and microsomes from male rats were intermediate. The ratio between the formation of deethylated and hydroxylated metabolites increased with the lidocaine concentration and at a lidocaine concentration of 10-4 M deethylation was the dominant oxidation type in all microsomes. The data suggest that the two spectral phases represent two binding sites of cytochrome P-450 each having a certain “catalytic specificity” - the high affinity catalyzing aromatic hydroxylation and the “low-affinity site” deethylation. This hypothesis is further supported by the observed differential effects of pH and MgCl2 concentration on the two types of oxidation.