Using heat pulses the apparent velocity of the transpiration stream was measured in apple (Malus sylvestris Mill.) grafted on French seedlings (M. baccata Desf.), and several cultivars of peach (Prunus persica Sieb. and Zucc.) and Japanese plum (Prunus triflora Roxb.) grafted on almond (Prunus amvgdalus Batsch.) rootstock. The trees varied in compatibility with their respective rootstocks, as established by visual observation and gross morphology of the zone of union. Velocity of the transpiration stream was measured in 3 regions of each tree: the rootstock stem, the zone of union, and the scion stem. In definitely defective unions velocity of the transpiration stream in the zone of union exceeded that in both rootstock and scion. The possible use of this method as a tool for identifying poor graft unions in orchards is evaluated.