A gas chromatographic method has been developed which enables accurate determination of a new antihypertensive agent, DU-717, in plasma. Accurate determinations are possible over a concentration range from 10 to 150 ng/mL of DU-717 in plasma at a relative standard deviation of 6.2%. The minimum detectable concentration is 1 ng/mL. Plasma levels of DU-717 in spontaneously hypertensive and normotensive rats following single oral administrations (10 mg/kg) have also been determined.