Within a controlled double-blind clinical trial the psychopharmacological effect of mepiprazole was tested against placebo in the treatment of neurotic inpatients. Some results of the recorded psychophysiological parameters (tremor, reaction time and flicker fusion threshold) as well as the scores on a symptom checklist for neurotic patients (post hoc divided into three scales by means of factor analysis: anxiety, depression and weaping) indicate a tranquilizing effect of mepiprazol. These findings are discussed critically with regard of methodological problems of testing tranquilizers in psychiatric hospitals.