Canopy transpiration of mature Jeffrey pine was compared in "mesic" and "xeric" microsites differing in topographical position, bole growth, and the level of drought stress experienced. Diurnal and seasonal course of canopy transpiration was monitored with thermal dissipation probes in 1999 and 2000. Mid-canopy measures of diurnal foliar stomatal conductance (gs) were taken in June and August in 1999. In early summer, there was little difference between trees in either microsite with regard to gs (55 mmol H2O m−2s−1), canopy transpiration (4.0 l h−1), and total duration of active transpiration (12 h >0.03 l h−1). In late summer, xeric trees had a lower daily maximum gs (by 30%), a greater reduction in whole canopy transpiration relative to the seasonal maximum (66 vs 79%), and stomata were open 2 h less per day than in mesic trees. Based on leaf-level gas exchange measurements, trees in mesic sites had an estimated 46% decrease in O3 uptake from June to August. Xeric trees had an estimated 72% decrease over the same time period. A multivariate analysis of morphological and tissue chemistry attributes in mid-canopy elucidated differences in mesic and xeric tree response. Mesic trees exhibited more O3 injury than xeric trees based on reduced foliar nitrogen content and needle retention in mid-canopy.