An electron-capture gas chromatographic method for the determination of isosorbide 5-mononitrate in human plasma using a capillary column is described. Isosorbide 5-mononitrate and the internal standard (isosorbide dinitrate) are extracted from the alkalinized plasma with ether. The lower limit of detection for isosorbide 5-mononitrate is 1 ng/mL of plasma.