Rats malnourished since birth and fed a protein-free diet for 2 wk showed almost undetectable 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.