The development of aryl hydrocarbon (benzo(a)pyren)hydroxylase (AHH), succinate dehydrogenase and glucose-b-phosphatase activities in the liver and the kidney and of liver weight and glycogen were investigated in 7-day-old rats treated i.p. with 1, 10, and 100 mg 3-methylcholanthrene (MC)/kg body weight. 3-MC highly specifically influenced the AHH activity, which was increased dose dependent. The higher the dose, the higher and the longer was the induction effect. The other biochemical parameters investigated were not influenced. At the maximum of the induction effect after pretreatment with the highest dose of 3MC no morphological alterations in the liver could be observed by light and electron microscopy. Therefore the 3-MC mediated AHH induction seems to be a suitable model for the examination of dose dependent inducer-inhibitor interactions in the intact animal.