Chlordiazepoxide (CDZ) was labeled with I-131 and investigated its radiopharmaceutical potential as a benzodiazepine receptor agent. Iodogen was used as iodination agent. Labeling yields were determined by radio-ITLC (instant thin layer chromatography) and paper electrophoresis. Labeling yield was approximately 90%. Purification was performed by Sep-Pak C-18 plus. Biodistribution studies were carried out by albino Wistar rats. I-131 labeled chlordiazepoxide which is the 62.2 GBq/mmol specific activity was administered by tail vein into the rats. Rats were sacrificed by ether narcotization at certain time intervals and the organs were removed. The brains were dissected into their components. Their activities were counted by a gamma-counter. The activity per gram tissue was calculated and time-activity curves were generated.