Microsomes from the liver of sea bass (Dicentrarchus labrax) were shown to hydroxylate lauric acid at subterminal positions. The cytochrome P-450 system converted lauric acid to several mono-hydroxylated metabolites including omega-1 hydroxylaurate, which was the major metabolite (44% of total products). In addition, omega-2, omega-3, omega-4 and a small amount (2.3%) of omega hydroxylaurates were found. Reaction products were identified using thin-layer chromatography (TLC) and gas chromatography/mass spectrometry (GC/MS). Oxidation reactions were dependent upon O2 and NADPH, and did not occur with boiled microsomes or in the presence of a mixture of CO/O2. Hydroxylation proceeded linearly up to 20 min at 28 degrees C for protein concentrations below 380 micrograms. Treatment of fish with benzo(a)pyrene (BP) (20 mg/kg) drastically increased xenobiotic metabolism (ECOD, EROD and BPMO activities), but no difference in laurate hydroxylase activity was observed between untreated and treated fish. Starvation strongly enhanced laurate hydroxylase activity, and resumption of feeding reduced by half this increase of activity. In all of the experiments we did not observe any modification of the regioselectivity of lauric acid hydroxylation by this microsomal in-chain hydroxylating system. We suggest that cytochrome P-450 enzymes involved in lauric acid and xenobiotics metabolism are regulated independently.