Water stress (LWP) was measured in honey mesquite, Prosopis glandulosa var. glandulosa [Torr.], trees by using pressure—chamber method to detect negative hydrostatic pressure in the xylem of leaf petioles. The method was sufficiently sensitive to measure short—term changes of ± 1 bar LWP throughout the diurnal priod. Mean differenes between sun—exposed and shaded leaves averaged 4.7 bars LWP on a clear day in May. Although the mean LWP for five trees ranged from 15.8 to 18.1 bars LWP, the interaction of exposure and tree was relatively minor in comparison with the large diurnal differences in LWP. Diurnal changes in LWP, often greater than 20 bars, were associated with changes in the evaporative potential of the atmosphere. Total daily water stress (ΣL) and net daily water stress (ΣL ‐ ΣL0) were calculated by integrating the area under a curve formed by the periodic measurements of LWP during the diurnal period. Net daily water stress was more sensitive to changes in environmental variables than other daily summaries. Throughout the range of available soil moisture there was a close relationship between net daily water stress and the integrated evaporative potential on a daily basis.