Widespread establishment of seedling aspen (Populus tremuloides) occurred in Yellowstone National Park (YNP) following the extensive1988 fires. Aspen stands occupy ∼2% of YNP and aspen stems are intensively browsed by native ungulates. Chemical composition, especially secondary compounds, may influence levels of herbivory and, hence, survival of aspen, but concentrations of such compounds in aspen in the northern Rocky Mountains are not known. Quantitative profiles of foliar nitrogen and secondary compounds (condensed tannins and the phenolic glycosides, salicortin and tremulacin) in aspen were assessed to address the following questions: (1) Do concentrations of secondary compounds differ between seedling and mature aspen stands? (2) Do concentrations of secondary compounds in seedling aspen differ between unbrowsed and artificially browsed seedlings? (3) Among mature aspen stands, do concentrations of secondary plant compounds differ among, (a) burned and browsed, (b) unburned and browsed and (c) unburned and unbrowsed stands? Concentrations of phenolic glycosides were significantly higher in seedlings than in mature stands, although condensed tannin concentrations and leaf nitrogen were higher in mature stands. Concentrations of leaf nitrogen and all secondary compounds were greater in unbrowsed seedlings than in seedlings subjected to simulated browsing. Concentrations of secondary compounds did not differ between mature aspen stands that were unburned regardless of whether they were browsed; however, burned stands (all of which were browsed) had significantly greater concentrations of secondary compounds and leaf nitrogen than the unburned stands. Results from this research suggest that foliar phenolic glycosides and tannins are not active defenses induced in response to browsing by large mammals. Rather, variation in levels between juvenile and mature ramets represents ontogenetic shifts in expression of defense, whereas variation between clipped and unclipped seedlings results from shifts in carbon/nutrient availability.