Phyto-psychopharmacological agents are extracts of plants with stimulating or calming effects on the central nervous system. Phyto-psycho-pharmacological agents are among the most commonly prescribed herbal medicines in Germany. The efficacy and harmlessness of some of the preparations have been established by high quality clinical trials. Between 1975 and 1992, a total of 34 clinical studies involving some 2326 patients were published on the effects of Ginkgo special extract EGb 761 and LI 1370; to date, 28 clinical trials in 2120 patients have been under-taken with alcoholic extracts of St. John's Wort. The therapeutic efficacy of kava and valerian extracts has been investigated in six and four controlled studies, respectively. In general, a high placebo effect is likely, which is why it is essential to include control groups in these studies. A considerable advantage over synthetic psychopharmacological agents is the low incidence of side effects, which in safety assessment studies is below 3%. The sharp increase in quality standards for clinical trials has meant that only a few preparations have undergone large scale testing programs in accordance with international guidelines. For other phyto-psychopharmacological agents, there is the danger that no further clinical trials will be undertaken due to the excessively high standards now demanded.