The cytochromes P-450 metabolize a wide variety of drugs, chemicals and carcinogens. Specific modulators or aryl hydrocarbon hyrdroxylase (AHH) are useful probes for the study of the multiplicity, diversity, and different catalytic properties of the cytochromes P-450. We have examined the effect of a number of flavones on catalytic activity of rat hepatic microsomes. The compunds may be classified according to their activating or inhibitory effect on the constitutive and 3-methylcholanthrene (MC) induced AHH. One class of flavones, typified by 7,8-benzoflavone, activates constitutive AHH and inhibits the MC-induced AHH. A second class, typified by maackiain, exhibits effects which are the reverse of 7,8-benzoflavone. The flavones we screened may prove useful in studies which characterize the cytochrome P-450 content of different tissues and should be useful in exploring the mechanism of action of these isozymes.