Five experiments were conducted on four species of fruit tree (plum,apple,walnut, peach) at four locations to determine the reduction of tree transpiration with decresing soil water availability, In all these experiments, control trees received an appropriate irrigation, while the water supply of the treated trees was suppressed for three to four weeks. In three experiments, a second drought period was imposed after a recovery period. Daily transpiration was estimated by sap flow measurements (heat balance method) and predawn leaf water potential (PLWP) was measured almost daily by using a presure chamber. relative transpiration (RT) of water deprived trees was therefore calculated all through the drought period. RT and PLWP were highly correlated, but the regression coefficients differed among the four species. When two consecutive drought periods were imposed, different relationship were observed in relation to the intensity and the duration of drought. Before the beginning of a drought period, the PLWP observed was not the same for the different species; these values were used as a normalising factor leading to a unique RT vs. PLWP function for all treaments and species, If the validity of such a normalisation is confirmed, it would greatly enhance the potential use of PLWP as a water stress indicator for irrigation scheduling and for irrigation trials.