In vitro metabolism of adenine, 9-methyladenine, and 9-benzyladenine using hepatic microsomes of hamster, mouse and rat was investigated. The results indicated that adenine was apparently not susceptible to microsomal N-oxidation. N-oxidation of 9-methyladenine was also not detected, whereas N-demethylation was observed with hepatic microsomes derived from hamster and rat but not from mouse. With 9-benzyladenine, both 1-N-oxide formation and N-debenzylation occurred with microsomes of all species in various amounts. N-Hydroxylation of the 6-amino group was not observed with any substrate in any species. Metabolic results are discussed in relation to chemical structure, electronic, lipophilic and steric factors.