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Browsing by ungulates is one cause of aspen decline in western North America. The goal of this study was to explore the influence of an herbivore�s nutritional state on aspen intake. Thirty-two lambs were penned individually and randomly assigned to two groups (16 lambs/group). Group 1 received freshly harvested leaves of an aspen stand with high content (AH) (21.2%) of phenolic glycosides (PG)� a chemical defense� whereas Group 2 received leaves of an aspen stand with low concentration (AL) (16.3%) of PG. For Experiment 1, half of the animals in each group were offered a ration of low (L1) crude protein (CP):energy (DE) ratio (CP/DE) (CP: 10.7%, DE: 3.13 Mcals), and the other half received a ration of high (H1) CP/DE (CP: 20.3%, DE: 3.13 Mcals). During Experiment 2, lambs were re-randomized and half of the animals in each group received a ration of high (H2) DE/CP (CP: 13.43% DE: 3.45 Mcals), and the other half received a ration of low (L2) DE/CP (CP: 13.33% DE: 2.52 Mcals). In Experiment 1, aspen intake was greater when animals received H1 than when they received L1 (p=0.02), and when they had AL than when they had AH (p=0.009). In Experiment 2, aspen intake was greater in L2 than in H2 (p=0.001). In summary, lambs under a basal diet high in protein ate more aspen likely due to the beneficial effects of protein on detoxification processes and to the need for additional energy intake. In contrast, a basal diet high in energy led to lower aspen intake likely because satiety induced by calories reduced appetite for less desirable �and defended� forages like aspen. Thus, herbivores grazing an understory with high protein/energy ratios are more prone to consume greater amounts of aspen than those animals grazing understories of greater energy and lower protein content.