7-Ethylbenz[a]anthracene (7-EBA) is a much weaker carcinogen than the 7-methyl analogue (7-MBA), and this difference may be based upon differences in the pathways by which the two compounds are metabolized and activated. In the present work, 7-EBA and the related 7 alpha- and 7 beta-hydroxyethyl derivatives (7-OHEBA and 7-beta-OHEBA) have been incubated with microsomes prepared from the livers of rats pretreated with 3-methylcholanthrene, the metabolites were extracted and purified by TLC, and the products present in the dihydrodiol band were examined by analytical HPLC. Metabolites were identified by comparison with authentic reference standards and by their chromatographic, UV, fluorescence, mass, and NMR spectral characteristics. The 7-EBA metabolites included the 1,2- 3,4, 5,6- 8,9- and 10,11-dihydrodiols, the 3,4- 8,9- and 10,11-dihydrodiols of 7-alpha-OHEBA, and three phenolic dihydrodiols that were not completely characterized. No 7-beta-OHEBA derivatives were detected as metabolites of 7-EBA. The results obtained so far have not revealed any qualitative differences in the routes by which 7-EBA and 7-MBA are metabolized in rat liver microsomal preparations.