Process for producing radiolabelled benzodiazepine receptor agents and composition containing the same

A process for producing a radiohalogenated 3-(5-cyclopropyl-1,2,4-oxadiazol-3-yl)-5,6-dihydro-5-methyl-6-oxo-4H-imida zo[1,5-a][1,4]-benzodiazepine wherein a precursor of the general formula (I) ##STR3## wherein R is bromine or iodine and R is a 7, 8, 9, or 10 substituent, is converted into a radiohalogenated 3-(5-cyclopropyl-1,2,4-oxadiazol-3-yl)-5,6-dihydro-5-methyl-6-oxo-4H-imida zo[1,5-a][1,4]-benzodiazepine of the general formula ##STR4## wherein R.sup.1 is a radioactive halogen, selected from radioactive iodine, radioactive bromine, radioactive fluorine and R is a 7, 8, 9 or 10 substituent, by performing a radiohalogenization reaction comprising exchanging the inactive halogen of the precursor (I) into a radioactive halogen at an elevated temperature in a halogen-free acidic medium. whereafter the end product (II) is separated from the precursor (I) by using chromatographic methods.