The development of aryl hydrocarbon (benzo(a)pyrene)hydroxylase (AHH), succinate dehydrogenase and G-6-Pase activities in the liver and kidney, and of liver weight and glycogen were investigated in 7 day old rats treated i.p. with 1, 10 and 100 mg of the carcinogen 3-methylcholanthrene (MC)/kg body weight. 3-MC specifically, influenced AHH activity, whose increase was 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 3-MC no morphological alterations in the liver could be observed by light microscopy and EM. 3-MC mediated AHH induction seems to be a suitable model for the examination of dose dependent inducer-inhibitor interactions in the intact animal.