Our studies are concerned with cytochrome P-450 content and 2-acetylaminofluorene N-and ring-hydroxylation by thiamine deficient rat liver microsomes. Incubation medium contained 100mM HEPES buffer pH 7.8, 2mM NADPH, 100mM KF, 100nM [¹⁴C]-AAF and microsomal protein. After 30min, incubation at 37℃, ring and N-hydroxy-9-¹⁴C-AAF formations are assayed by radioactivity measurements after paper chromatography separation. The concentration of cytochrome P-450 is elevated in microsomes from rats fed the deficient diet. It appears that the thiamine deficient rat liver is capable of producing effects on the drug hydroxylation enzymes.