Diazepam belongs to the 1,4-benzodiazepine group of compounds, which are used for medical purposes as tranquilizers and antidepressants. This Bulletin describes the determination of diazepam in tablets and body fluids (blood, serum, urine) by means of differential pulse polarography. If a Britton-Robinson buffer pH = 2.8 with a methanol volume fraction of 20% is used as the supporting electrolyte then a pronounced reduction peak is obtained at -0.73 V; this allows diazepam concentrations even below 0.05 µg/mL to be determined in blood. The necessary sample preparation steps are also dealt with in this Bulletin.