A specific and quantitative assay for determining nanogram levels of nitroglycerin in plasma was developed. The method involves stabilization of the drug in plasma with silver nitrate, followed by multiple extraction using purified hexane. Isosorbide dinitrate is added as the internal standard. The hexane extract is subsequently concentrated and injected into a glass column packed with 3% SP-2401 on 100--120-mesh Supelcoport at 140 degrees. A 63Ni-electron-capture detector gives a linear response over the range of 0.1--50 ng of nitroglycerin/ml of rat plasma. From spiked samples, the procedure gave a recovery of about 90%. There was little or no interference from the isomeric glyceryl dinitrates and endogenous compounds in rat or human plasma.