Abstract Phenobarbital, 1,1,1-trichloro-2,2-bis( p-chlorophenyl)ethane (DDT), benzpyrene, 3-methylcholanthrene (3-MC) or 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) were administered i.p. for 1 or 3 days to genetically “responsive” (C57BL/6J) and genetically “non-responsive” (DBA/2J) mice. 3-MC or benzpyrene stimulated aryl hydrocarbon hydroxylase (AHH) activity in C57BL/6J (B6) mice but not in DBA/2J (D2) mice. TCDD induced AHH activity in both B6 and D2 mice. Time-course studies showed that in the first 12 h after a single injection of 3-MC to B6 mice there was no shift in the reduced cytochrome P-450-CO complex absorption spectra from 450 to 448 nm, although AHH activity increased 4–5 times over (above) that of the control group. The relationship between induction of AHH activity by polycyclic hydrocarbons in B6 mice and the concomitant synthesis of cytochrome P-448 is discussed.