Abstract The ontogeny of rat liver nuclear aryl hydrocarbon hydroxylase (AHH) was studied. AHH which was barely detectable in 18–20 day fetal rat liver nuclei increased rapidly post-partum reaching a value which was over 200 times greater than the fetal liver specific activity. Nuclear AHH was induced upon administration of 3-methylcholanthrene (3-MC) to rats. The level of induction was dependent upon the age of the rat. Thus, the highest induced nuclear AHH specific activity was observed in the 3–4-week-old rat, i.e., 50–70 g in weight. The greatest fold increase in nuclear AHH after 3-MC administration was observed in the 6–8 g, or 1–2-day-old rat. Fetal rat liver nuclear AHH was also induced after 3-MC administration to the dam. The basal and induced levels of AHH were compared in nuclei and microsomes from 1–2 and 10–12-day-old rats. In the former, the ratio of the microsomal to nuclear AHH was 1.4 and 1.8 under basal and induced conditions, respectively.