35S-L-cysteine was fed to a mature leaf of 3-year-old beech trees via a flap. After 1 to 4 h the distribution of 35S-radioactivity was analysed in the leaves as well as the bark and wood of the trunk and the main root. Transport of 35S out of the fed leaf amounted to 0.3–1.2% of the total 35S taken up. The branches of the trees did not act as sink organs for the exported radioactivity. The main portion of the 35S-radioactivity transported out of the fed leaf was found in basipetal parts of the trunk. Only a small portion of 35S-radioactivity was transported in acropetal direction. The distribution of the 35S-radioactivity within the trunk showed a higher portion of 35S in the bark than in the wood. In both tissues, bark (70 to 80%) and wood (60 to 70%), the 35S was predominantly found in the HCl soluble fraction. However, 35S-cysteine, the compound fed to the leaves was not exported out of the fed leaf. Along the trunk 35S-cysteine was neither determined in bark nor in wood sections. The only low molecular mass S-compounds found was 35S-glutathione (GSH). The 35S-sulphate detected in bark and wood origined from cysteine oxidation in the leaf tissue and from contamination of the 35S-cysteine feeding solution. The ratio of GSH to sulphate decreased with increasing distance from the fed leaf. Apparently, 35S-radioactivity was transported as sulphate and GSH in the phloem in basipetal direction, but GSH was removed preferentially out of the phloem along the transport path. 35S-radioactivity exported out of the phloem and transported into the wood of the trunk was not retranslocated in the xylem. It may therefore be assumed that part of the 35S translocated was stored in ray cells, medullary sheath cells and/or pith parenchyma cells. Girdling experiments in which the bark of the trunk was peeled off basipetal to the branch containing the fed leaf support these assumptions.