Résumé : After a preliminary encouraging trial of mepiprazol (EMD 16.923) on 8 patients, a double blind cross over study of mepiprazol and diazepam was performed in order to compare their effect in the treatment of neurotic disorders, particularly on various forms of anxiety. The study was performed on 24 patients of both sexes, 27 to 61 years old, who received randomly one or the other medication alternating for one month, then the other during one month, a placebo being given for 2 weeks between the two active therapeutic phases. A uniform daily dosage of 3 x 5 mg was chosen for the two compounds. The clinical evaluation was performed at the end of the therapeutic phases, i.e. on days 0, 30, 45 and 76, by the psychiatrist according to a list of symptoms and by the patient according to a questionnaire, established after the (Befindlichkeitsskale) of von Zerssen. The two compounds were active on anxiety symptoms to a highly significant degree, but no significant difference was shown between them. The tolerance was sufficient in every case, diazepam being slightly more sedative than mepiprazol which affected remarkably little both vigilance and psychomotricity.