Arguments are given for a ferredoxin-mediated reduction of Tc04-, preponderantly into extractable Tc(V) complexes, by illuminated, broken chloroplasts. Photosynthetic O2-and NADP-reduction competitively inhibit Tc incorporation. As for 02, the reaction can be stimulated by the auto-oxidizable electron acceptor methyl viologen. Furthermore Tc04-can function as terminal acceptor in the diaphorase reaction, with NADPH as electron donor.