Thermal dissipation probes were used to determine sap flow velocity in 125-year-old, 25-m-tall lodgepole pine trees at 1650 m on the interior plateau in British Columbia Canada. During 2000 and 2001, measurements began in the spring and terminated when the snow pack began to develop. Stand density and diameter distribution data were used to convert sap flow to an estimate of transpiration. Measurements of soil water content using time domain reflectometery and of rainfall interception were used in determining the stand water balance. Sap flow showed a large variation between trees and in some cases between north and south quadrants of the same tree. Maximum values were about 1400 cm3/15 min. Differences between trees were related to the size of the tree crown. Flow was correlated with PAR and vapour pressure deficit. Maximum daily tree transpiration was estimated to be 1.7 mm/d (plant area index is about 2.5). Daytime air temperatures below 0C resulted in a termination of sap flow and it recovered slowly when subsequent days were warmer. Soil water content data indicated that the tree transpiration was about 60% to the evaporation from the ecosystem when the canopy was dry.