The transport of sugars produced by photosynthesis through the phloem, from leaves to various parts of the plant, plays a major role in the carbon budget of forests, but the actual mechanisms that control the rate of phloem transport remains unsolved. A flux of water from xylem to phloem tissues during the day is thought to play an important role in maintaining the hydrostatic pressure gradient which is responsible of the mass flow in the phloem sieve cells, but no study has allowed a direct observation of this water exchange so far.In this study, the exchange of water between the xylem and phloem of hinoki cypress was traced by analysing the isotope composition of water extracted from leaves, sapwood and bark of branches of trees which were labelled by watering the three trees with deuterium oxide water. The isotope composition of sapwood water and bark water increased the day after labelling almost at the same time before it increases in leaf water. This strongly supports our hypothesis that there is a flux of water from the xylem to the phloem during the day, and that secondary xylem and phloem are hydraulically connected.