The rate of photosynthetic carbon fixation and of root and shoot xiv respiratory carbon use was measured in the laboratory and in the field (shoots only) for Agropyron desertorum (Fisch. ex Link) Schult. and Agropyron spicatum (Pursh) Scribn. and Smith. The rate of respiratory carbon use of the root system declined within hours of the shading or defoliation of the shoot system, resulting in as much as 60% reduction in specific rate of root respiration. The mean whole-plant growth efficiency (the ratio of whole-plant net carbon gain to gross photosynthetic carbon fixation) in full irradiance in the laboratory was 0.53 and was reduced both by shading and defoliation. The mean conversion efficiency was o. 70 and o. 73, and the mean maintenance coefficient 20°c was 10.8 and 9.9 mmol C mol C-1 d-1 for A. desertorum and A. spicatum, respectively. These maintenance coefficients are lower than previously reported for fast growing crop plants. The rate of respiratory carbon use and the dynamics of labile carbon compounds were simulated both for intact plants and for plants regrowing following defoliation. The partitioning of assimilates between root and shoot was explicitly modeled to make the separate simulation of root and shoot respiration possible . The simulated daily net mobilization of labile carbon compounds exceeded carbon input from photosynthesis for only the first one-to-two days of regrowth, depending on the severity of the defoliation. The instantaneous rate of respiratory carbon use of the shoot system in the field during short-term light exclusion during the day was higher than the rate at the same temperature during the subsequent night. The Qio of shoot respiration was estimated to be 2.1-2.2. The mean growth efficiency in the field for the shoots only was 0.65 for sunny days. This efficiency was higher than the whole-plant growth efficiency in the laboratory because root respiration was not measured in the field .