1. Stable extract of Hypericum perforatum L. (St. John's wort) containing Hyperf orin, characterized in that the Hyperforin is stabilized against decomposition or degradation by means of a stabilizer selected from the group consisting of anti-oxidant and oxygen binding stabilizers and reducing agents. 2. Stable extract according to claim 1, characterized by a content of a stabiliz er selected from the group of organic thiol compounds and ascorbic acid and derivatives thereof in an amount sufficient for stabilizing the Hyperforin. 3. Stable extract according to claim 1, characterized in that the stabilizer is present in a concentration of 0.01% to 5%, based on the extract. 4. Stable extract according to claim 1, characterized in that the stabilizer is present in a concentration of 0.2% to 1%, based on the extract. 5. Stable extract according to claim 1, characterized in that the stabilizer is cysteine. 6. Stable extract according to claim 1, characterized in that the stabilizer is glutathione. 7. Stable extract according to claim 1, characterized in that the stabilizer is ascorbic acid. 8. Stable extract according to claim 1, characterized in that the stabilizer is a fatty acid ester of ascorbic acid. 9. Process for the preparation of a stable Hyperforin-containing extract wherein a crude herbal drug (Hypericum perforatum L.) is extracted with pharmaceutical conventional anorganic or organic solvents or mixtures thereof, with the proviso that the sol vents are no oily extraction agents and wherein a stabilizer selected from the group of organic thiol compounds, ascorbic acid and derivatives thereof is added, optionally during or after the p reparation of the extract, in an amount sufficient for stabilizing the Hyperforin and wherein a dry extract is obtained from the thus obtained liquid extract. 10. Process according to claim 9, characterized in that a fresh crude herbal drug (Hypericum perforatum L.) is extracted. 11. Process according to claim 9, characterized in that a dried crude herbal drug (Hypericum perforatum L.) is extracted. 12. Process according to claim 9, characterized in that cysteine is used as stabilizer. 13. Process according to claim 9, characterized in that glutathione is used as s tabilizer. 14. Process according to claim 9, characterized in that ascorbic acid is used as stabilizer. 15. Process according to claim 9, characterized in that a fatty acid ester of as corbic acid is used as stabilizer. 16. Process according to claim 9, characterized in that the stabilizer is added in a concentration of 0.01% to 5%, preferably 0.2% to 1%, based on the extract. 17. Process according to claim 9, characterized in that a solvent is used for extraction, the oxygen content of which is low or was considerably reduced. 18. Process according to claim 9, characterized in that a solvent is used for extraction selected from the group of aqueous ethanol, aqueous methanol, alkane having about 5-8 degree C-atoms, and liquid or supercritical carbon dioxide. 19. Process according to claim 9, characterized in that the stabilizer is added after drying the extract solution. 20. Process according to claim 9, characterized in that the stabilizer is added only at the stage of the finished pharmaceutical composition together with conventional pharmaceutical additives. 21. Process according to claim 9, characterized in that the process is carried out by excluding light and/or oxygen. 22. Pharmaceutical composition containing an extract according to any of claims 1 to 8 and conventional pharmaceutical additives for the treatment of depressions and psychovegetative disorders.