Sieve tube sap was collected either from the severed stylets of Tuberolachnus salignus (Gmelin) or via incisions made into the phloem of small willow trees or potted cuttings. Measurements of the osmotic potential (O.P.) of sap samples showed a gradient to exist in the presumed direction of assimilate transport, ie from apex to base of the stem.In most experiments samples of phloem tissue were taken after the collection of sieve tube sap, the water potential of these pieces of tissue being measured in a psychometer. Although a water potential gradient existed in the opposite sense to the O.P. gradient in the sap (lowest water potential at the apex of the stem), the difference between O.P. and W.P. indicated the turgor of the sieve tubes to be higher at the apex than at the base of the stem. The magnitude of the turgor gradient measured in this way lay between 0.5 and 2.7 atm m(-1).In other experiments severed stylets only were used to determine whether a hydrostatic gradient can exist in willow sieve tubes. After measurement of flow rates from stylets sited at the apex and base of willow stems, the Poiseuille expression was used to calculate the pressure at the point of stylet puncture. These experiments gave values for the pressure gradient (in the presumed direction of assimilate flow) of between 1.9 and 4.7 atm m(-1).