The synthesis and biological evaluation of three classes of chain-modified derivatives of (+)-EHNA are described. Among the 5', 6'-unsaturated derivatives, the Z-isomer was the most potent inhibitor of adenosine deaminase (ADA) but 3-fold less active than (+)-EHNA. Several 9-aralkyladenines (ARADs) have been prepared, and their inhibitory activity was determined. A minimum of two carbon atoms separating the aromatic ring from the adenine-bearing carbon (C-3') was found to be essential for ADA activity equal to or slightly greater than that of (+)-EHNA. Finally, replacement of the C-5' carbon with an oxygen resulted in reduced potency.