Induction of CYP1A1 is one of the best characterized responses to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). EROD activity has been used as an enzymatic marker for CYP1A1 following TCDD treatment. Enzymatic markers for the induction of CYP1A2 by TCDD are not as well characterized. The present study examines the relationship between CYP1A1 and CYP1A2 protein and the corresponding enzymatic markers. Induction of hepatic ethoxyresorufin O-deethylase (EROD) activity and methoxyresorufin O-demethylase (MEROD) and acetanilide 4-hydroxylase (ACOH) activity (both markers for CYP1A2) were analyzed in 8-wk-old male and female Fischer 344 rats treated orally with either 0, 0.1, 0.3, 1.0, or 3.0 micrograms TCDD/kg. There were no sex differences in basal EROD or ACOH activity. MEROD activity was significantly greater in control males than in control females. Significant induction of EROD activity in females occurred at slightly lower doses of TCDD compared to males (0.1 vs. 0.3 micrograms/kg, respectively); however, a greater absolute and a larger fold induction of EROD activity was seen in males compared to females at all doses tested except 0.1 micrograms/kg. EROD activity did not attain a maximum in either sex. Similarly, MEROD activity was induced at lower doses of TCDD in females than in males (0.1 vs. 0.3 micrograms/kg, respectively). MEROD activity was maximally induced at 0.3 micrograms/kg in males. In females, MEROD did not attain maximum induction at the doses tested. ACOH activity was induced at doses as low as 0.3 micrograms/kg in both sexes, and the dose-dependent increases in activity were equivalent in males and females. Both ACOH and MEROD activity correlated well with CYP1A2 levels as determined by Western blot analysis, although there was a greater fold induction of protein than either MEROD or ACOH. Although MEROD and ACOH are both markers for the same response, MEROD activity may be a more useful marker because it is the quicker and more sensitive of the two assays.