Rats malnourished since birth and fed a protein‐free diet for 2 wk showed almost un‐detectable levels of liver microsomal aryl hydrocarbon (benzo[a]pyrene) hydroxylase. Treatment with benzo[a]pyrene rapidly enhanced this activity to levels higher than those observed with untreated normal rats. The carbon‐monoxide‐reduced cytochrome P‐450 spectral peak was shifted from 452 nm in malnourished untreated rats to 448 nm in malnourished benzo[a]pyrene‐injected rats and resulted in increases in the intensity of several microsomal protein bands (MW range 46,000–60,000) separated by gel electrophoresis. Malnourished rats then fed with a protein diet exhibited an important increase in aryl hydrocarbon hydroxylase activity, an increase in the intensity of microsomal protein electrophoretic bands (MW range 46,000–60,000), and a shift of the carbon‐monoxide‐reduced cytochrome P‐450 spectral peak from 452 nm to 450 nm. These results suggest that alterations in cytochrome P‐450 species related to benzo[a]pyrene metabolism might explain the modulation of this activity by nutritional effects.