AbstractThe modulation of the phase I and phase II biotransformation enzymes upon treatment with tobacco extract (TE) andN'‐nitrosonornicotine (NNN) was investigated using male Sprague‐Dawley rats fed differential protein diets. It was observed that the animals fed a low protein diet showed an overall decrease in the basal levels of hepatic and pulmonary phase I and II enzymes. TE and NNN significantly decreased the detoxifying system in the low‐proteinfed animals. Animals fed 20% protein, however, showed significant increases in glutathione and glutathioneS‐transferase upon treatment. Furthermore, TE and NNN treatment brought about a significant depletion in the hepatic pool of vitamin A with a concomitant increase in the vitamin C