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Chemical, mechanical and phenological mechanisms by which plants avoid or minimize herbivory have received considerable attention in the last decade (e.g. Janzen 1969; Rosenthal 1977; Cates and Orians 1975; Janzen et al. 1976; Stiles 1977). Much less work has been directed towards an understanding of how plants tolerate heavy herbivore pressure. Plants of different life form often differ in tolerance to defoliation. Plant growth form influences the capacity to reestablish foliage following defoliation, such as by the protection or redundancy of apical meristems, or possession of active basal  intercalary meristems (Dahl and Hyder 1977). Life forms that are tolerant of herbivory also possess characteristics such as higher photosynthetic rates, reduduced foliage longevity, a low proportion of reproductive shoots, and faster rates of leaf replacement (Branson 1953; Archer and Tieszen 1980).  Striking differences in tolerance of herbivory by species of the same growth form and similar phenological characteristics seem less explicable. Therefore, we undertook the comparitive study of two Agropyron bunchgrasses A. spicatum (Purch) Scribn. and Smith and A. desertorum (Fisch. ex Link) Schult. which differ markedly in their tolerance of grazing.