An apparatus is described for the measurement of water flow in shoots of woody plants by a steady-state heat-flow method. Two thermocouples are set up on either side of a heating element inserted into the stem. The temperature difference between each of these thermocouples and the surrounding air is measured. Theoretical considerations show that the logarithm of the ratio of this difference in the ‘downstream’ to that in the ‘upstream’ position gives a measure of sap flow-rate. The apparatus provides a system that can be used in field or laboratory conditions to follow water transport in the leafy shoot over periods of at least 24 h.