Atriplex confertifolia and Eurotia lanata plants were studied in the field and laboratory under controlled conditions of temperature and vapor pressure deficit with uniform wind speed and light intensity and various levels of water stress. Multiple regression analysis indicated that under both field and laboratory conditions, vapor pressure deficit and water stress were significant factors influencing transpiration. Phenological stage was also a major factor in the transpirational relationships of both species in the field. Transpiration rates under comparable conditions tended to be higher for Eurotia than for Atriplex on a leaf dry weight basis with less pronounced differences on a leaf area basis. However, in the latter part of the summer, transpiration rates of Eurotia decreased markedly while Atriplex tended to maintain active transpiration. Under laboratory conditions both species exhibited transpiration at plant moisture stress values as great as 115 bars.