We evaluated in situ the effects of long-term grazing (>100 y) on the morphological (i.e., shoot height, leaf blade length, width and angle) and physiological (i.e., gas exchange and water relations) responses of the dominant perennial grass species from arid lowlands and more mesic uplands of the Pryor Mountain Wild Horse Range (PMWHR) in Montana. Pseudoroegneria spicata, the most abundant grass in the lowland communities, had shorter vegetative shoot heights and leaf blade lengths and narrower leaves in plants from grazed than long-term ungrazed sites. Similarly, vegetative and reproductive shoot heights of Festuca idahoensis and Elymus lanceolatus, common upland grass species, were shorter in plants from grazed than ungrazed sites. Leaf lengths of these upland grasses also were shorter and less erect in plants from grazed sites than ungrazed sites. The physiological responses of the dominant grasses to grazing were not consistent between species or sampling dates. Overall, photosynthetic rates, stomatal conductances and xylem pressure potentials were the same in over 65% of the comparisons between plants from grazed and ungrazed sites and were higher in grazed sites in only 11 to 22% of the grazed-ungrazed comparisons. Collectively, our results indicate that long-term grazing of grasses by wild horses and other herbivores in the PMWHR has resulted in morphological modification, but has not substantively altered physiological function.