During 1980-81, studies with crested wheatgrass (Agropyron desertorum) and fourwing saltbush (Atriplex canescens) were conducted in controlled environment growth chambers as well as under field conditions to achieve the following objectives: 1. To determine the effect of nitrogen fertilizer on the water use efficiency. 2. To determine the effects of various temperature, water stress and nitrogen treatments on the productivity, nitrogen content and carbohydrate reserves. 3. To determine the effects of N fertilization on fall and spring regrowth. Crested wheatgrass and fourwing saltbush plants were maintained in three growth chambers for 60 days under three temperature regimes (11/7, 19/7 and 27/7 C), two soil moisture stress regimes (-0.3 bars and -15 bars) and three N fertilizer levels (0, 50 and 100 kg of N/ha). During the study, tranpiration and plant biomass data were recorded. During the first week of September, 1980, crested wheatgrass and fourwing saltbush pastures at Nephi, Utah, were subjected to three nitrogen fertilizer levels (0, 50 and 100 kg N/ha). After 60 days the fall regrowth was clipped. In the first week of June 1981 spring regrowth of both species was measured. In the fall of 1981, a second experiment was laid out at Nephi where crested wheatgrass and fourwing saltbush plants were subjected to three soil moisture regimes (dry, medium and wet) and three nitrogen fertilizer levels. At the end of a 60 day study period, dry matter yield, root distribution, water content and soil samples at different incremental soil depths were collected. Under controlled environment conditions, the water use efficiency of both species was six percent more with the application of a moderate amount of nitrogen (50 kg/ha). A high temperature regime (27/7 C) and a high water stress regime (-15 bars) increased the water use efficiency of plants by eight and six percent respectively. Results of the growth chamber experiment revealed that nitrogen fertilization had a significant effect on plant biomass, nitrogen percent and total nonstructual carbohydrate reserves of crested wheatgrass and fourwing saltbush. The data further suggested that nitrogen fertilization can substitute for the adverse effects of low temperature and low soil moisture on plant growth. Nitrogen fertilization during fall increased plant biomass, nitrogen percent and total nonstructural carbohydrate reserves in crested wheatgrass and fourwing saltbush. Fall fertilization did not reduce spring regrowth. It is inferred that under limited soil moisture and low temperature during the fall growing season, a moderate amount of nitrogen fertilizer (50 kg N/ha) may increase the forage availability and water use efficiency of crested wheatgrass and fourwing saltbush to the level of plants maintained at moderate temperature and adequate soil moisture. Nitrogen fertilization (50 kg N/ha) of crested wheatgrass and fourwing saltbush during fall does not reduce plant nitrogen percent or carbohydrate reserves which may limit spring regrowth.