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Browsing by ungulates is one cause for aspen decline in western North America. The goal of this study was to explore the influence of plant chemicals (protein, aspen bark, phenolic glycosides [PG] and condensed tannins) on aspen intake and preference by sheep. Thirty two lambs were penned individually and randomly assigned to 4 molasses-based supplements (n=8) in a factorial design with repeated measures during three trials of 10 d each: (1) High-protein (HP-40% canola meal; 20% CP), (2) Condensed tannins (CT-6% quebracho tannins), (3) aspen bark (AB-25% aspen bark) and (4) Control (100% molasses). Supplements were fed at a rate of 1.42 Mcal of digestible energy/day from 0700 to 0900 and then lambs were offered freshly cut aspen leaves collected from aspen stands with high (13.2%; Trial 1) and low (7.9%; Trial 3) PG concentrations. During Trial 2, lambs were offered aspen leaves (33.8% PG), Utah pea, and smooth bromegrass. Refusals were collected after 2 h and a basal diet of tall fescue hay was offered until 1800. Aspen intake was greater when leaves had low concentrations of PG (5-7 vs. 2-3 g DM/Kg metabolic body weight [MBW]; P < 0.05). Supplements HP and CT led to higher intakes of aspen in Trials 2 and 3 (4 to 7 g/Kg MBW range), whereas supplement AB promoted the lowest intake of aspen (2 g/Kg MBW) during Trial 1 (P < 0.05). In summary, chemical defenses like PG in aspen bark and leaves depressed aspen intake by sheep but HP and CT had the opposite effect likely due to the beneficial effects of protein (e.g., rumen degradable protein, escape protein from CT-protein complexes) on detoxification processes.