The status of hepatic mixed function oxygenase system (MFOS) was investigated in rats fed spice principles: capsaicin, piperine and curcumin, as well as spices: cumin, ginger, fenugreek, cinnamon, asafoetida, mustard and tamarind at two dietary levels each. Liver microsomal cytochrome P450-dependent aryl hydroxylase was generally stimulated by these spice principles and spices. Cumin, ginger and fenugreek also stimulated the levels of cytochrome P450 and cytochrome b5 and cumin and tamarind stimulated N-demethylase activity. NADPH-cytochrome c reductase and glucuronyl transferase activities, however, remained unaffected by the spices tested.