The Dimroth rearrangement of 1-ethyladenine (6) to give N6-ethyladenine (11)(91% yield) was accompanied with unusual hydrolytic deaminations to produce hypoxanthine (8)(2%) and 1-ethylhypoxanthine (14)(2%), when carried out in 0.2N aqueous NaOH at 100°C for 7h. Probable pathways leading to these by-products are discussed on the basis of the results of alkaline hydrolysis of 5-amino-N'-ethylimidazole-4-carboxamidine dihydrochloride (5), which yielded both 5-aminoimidazoli-4-carboxamide (9) and 5-amino-N-ethylimidazole-4-carboxamide (12). For structural identification, 14 was alternatively synthesized from inosine (17) through 1-ethylinosine (16), and 12 was synthesized from 16 through 5-amino-N-ethyl-1-β-D-ribofuranosylimidazole-4-carboxamide (15). Comparison of the reaction rates in the Dimroth rearrangements of 6·HClO4 and 1-ethyl-9-methyladenine perchlorate [1·HClO4 (R1=Et; R2=Me)] in H2O at pH 6.92 and 8.70 (ionic strength 1.0) at 70°C has revealed that nonsubstitution at the 9-position decreases the rearrangement rate by a factor of 4-30 under these conditions.