We demonstrate a method for examining water flow and distribution within heterogeneous systems by means of MRI and deuterium oxide (D2O)/water (H2O) contrast. In this demonstration a piece of a pine tree was used. In pine xylem, water flows in tube-like dead cells, that is, tracheids, which are about 10-40 μm in diameter and 1-4 mm in length. Water flow in tracheids of a piece of a pine branch was studied by means of D2O/H2O contrast obtained with MRI at 1.5 T. D2O flowing through the object caused reduction of signal when 1H detection was used. Observed flow velocity in the compression wood was about one-third of that in the tension wood, the former having a smaller average cell lumen size than the latter. After the signal in the entire cross section was reduced to its minimum, the experiment was renewed with distilled H2O. Flow of the H2O and hence replacement of D2O through the wood resulted in the return of the signal. This study demonstrates the dependence of the flow velocity on the cell lumen size. The results suggest that D2O/H2O contrast obtained with MRI is an effective tool for studies of water dynamics within heterogeneous systems. © 2001 Academic Press.