Snowshoe hare (Lepus americanus) feeding preferences for Alaskan green alder (Alnus crispa) are governed by the concentrations of two deterrent secondary metabolites, pinosylvin and pinosylvin methyl ether. For instance, the preference of mature internodes over juvenile internodes was correlated with about a threefold increase in the levels of these compounds in the juvenile form. During the last year of the study, however, the levels of these compounds dropped below the threshold of avoidance in both types of internodes, resulting in nondiscriminatory use by hares even though the relative levels of these metabolites remained the same between the juvenile and mature form internodes. These conclusions are strongly supported with feeding bioassays using pure pinosylvin, pinosylvin methyl ether, and other less active secondary metabolites found in alder.