The authors, stimulated by the results of transplantations of embryonic nervous tissue under experimental conditions and by findings on the biochemical background of some manifestations of mental diseases, dared to use for the first time transplantation of portions of human embryonic brain in the treatment of schizophrenia. Before the experiment they evaluated the bilateral xeno as well as autotransplantations in ten cats. Under clinical conditions they used transplantation of embryonic nervous tissue in two patients with the acute and chronic form of schizophrenia. The operation was performed, using Mundinger's stereotactic apparatus controlled by a computer. The tissue from the central portion of the embryonic brain from the 5th and 7th week of gestation with a high serotonin content was deposited bilaterally in the septal area in the recipient's brain. They recorded improvement in particular in a case of acute schizophrenia. By monitoring the foci of transplanted tissue by computed tomography and magnetic resonance, they provided evidence that embryonic nervous tissue is incorporated in the recipient's brain without reverting into malignant growth and that it is not rejected by the immune system of the recipient.