The effects of the psilocybin, a psychomimetic substance, on mental junctioning were investigated in normal volunteers as well as in schizophrenics. The disturbances induced constitute a psychoneurotoxic syndrome--"a state of drunkenness"--of about four hours duration which develops in three distinct phases. The basic mental symptoms of this syndrome consist of disturbances of the apperception, sensory perception and emotion. A moderate impairment of egofunctioning or reality appraisal and an inability to integrate different mental processes are observed. The psychomotor behavior is mainly harmonized to the prevailing emotional state and to the experiences caused by perceptual alterations, in a lesser degree. These changes, according to our observations, are more severe and more "psychotic-like" in schizophrenics rather than in normals. Psychopathological analysis of these changes proves that the whole syndrome cannot be considered as related to the spontaneously triggered functional psychoses or to the organic ones and, therefore, the term "model-psychosis" according to our opinion, is unsatisfactory.