The biochemical mechanisms that limit voluntary intake of one-seed juniper by browsing ruminants are not well understood. Twelve Rambouillet ewes (78 ± 2.3 kg BW) and 12 Boer-Spanish does (54 ± 1.4 kg BW) were used in a split-plot sequence design to investigate the effects of adding protein, quebracho tannins (QTs), and polyethylene glycol (PEG) on one-seed juniper intake and preference and on ruminal VFA and plasma AA concentrations. Animals received sudangrass hay and isoenergetic basal diets (4 does and 4 ewes/diet) with either rumen degradable (RDP) or rumen undegradable (RUP) protein (12.5% CP) or no additional protein (control, 5% CP) during four 15-d periods. The control, RDP, and RUP diets were considered the main plot, whereas the four supplement treatments were analyzed as the split-plot effects. Period 1 allowed for adaptation to basal diets and served as a baseline phase. In periods 2-4, animals were offered juniper leaves and twigs (period 2), juniper plus QT (10% of basal diet; period 3), and juniper plus QT plus PEG (50 g/animal; period 4). Juniper intake by sheep and goats was not affected (P > 0.88) by RDP or RUP when animals were first exposed to juniper, but marginally increased (P < 0.10) in period 4 (QT + PEG) regardless of the basal diet. Prior exposure to juniper did not affect (P = 0.61) the preference ratio for juniper, but goats had higher preference ratio for juniper (P < 0.01) when receiving PEG (period 4). Concentrations of total VFA tended to increase in sheep (P = 0.10) and goats (P = 0.14) fed protein supplements and molar proportions shifted toward acetate for goats fed RDP and RUP (P = 0.07) and to butyrate for sheep fed RDP (P = 0.01). Initial juniper exposure (period 2) elevated concentrations of acetate, propionate, and butyrate (P < 0.01), but the effect was extinguished in periods 3 and 4 with addition of QT (P < 0.05). Supplementation with PEG transiently mitigated the depressor effect of QT on acetate, propionate, and butyrate concentration at 12 h post juniper feeding (P < 0.01). RDP in goats and to a greater extent RUP in sheep increased plasma concentrations of various AA, especially the branched chained Val, Ile, and Leu (P < 0.05). Plasma concentrations of several AA, including Met, Cyst, Glu, Gly, Gln, Asn, Thr, Ser, and Phe, decreased with ingestion of juniper (period 2) and juniper plus QT (period 3). Concentrations of some AA that were depressed by ingestion of juniper and QT were partially restored with supplemental PEG (P < 0.05). Protein, terpenes, tannins, and PEG interacted to influence rumen VFA and plasma AA, which were related to intake of juniper and basal diets.