The compositions of the volatile terpenes in steam-distilled oils from new growth and year-old needles of browsed and unbrowsed Douglas-fir trees were compared by gas chromatographic analysis. Levels of specific compounds did not differ significantly between browsed and unbrowsed trees in the young growth at bud burst, but differences appeared as the young growth matured. The browsed trees were significantly higher in α-pinene, -pinene, cis-ocimene, fenchyl alcohol, α-terpineol, and an unidentified sesquiterpene and lower in sabinene, α-terpinene, terpinolene, -terpinene, terpinen-4-ol, and α-humulene at one or more sampling times than unbrowsed trees. None of these differences were statistically significant in a smaller sample of trees 3 years later. Browsed trees had higher levels of microbial promoters and lower levels of inhibitors of rumen fermentation processes than the unbrowsed trees, but they did not differ in in vitro fermentability by deer rumen microbes, nor in nitrogen, crude fiber, crude fat, total sugar, sucrose, or moisture content. Significant variations among pairs of trees (sites) were noted for a few terpene components, in vitro fermentability, and crude fat. Although terpene concentrations in browsed and unbrowsed trees showed considerable variation, the levels of the terpenes that promoted rumen microbial fermentation averaged higher in browsed than unbrowsed trees while those which inhibited fermentation averaged lower. However, overall in vitro fermentability of browsed and unbrowsed trees did not differ significantly. Forest Sci. 26:179-193.