1 A gas chromatographic procedure for the determination of viloxazine in therapeutic concentrations in human plasma, with use of a flame ionization detector, is described. 2 The drug is extracted at pH 12 into hexane, back extracted into HCl, re-extracted into hexane after alkalinization of the HCl and derivatized with acetic anhydride. 3 An imipramine-butriptyline mixture is used as an internal standard. The coefficients of variation for a concentration of 0.75 microgram/ml are 9.4% and 13.1% for within day and day to day precision respectively. For a 1.75 micrograms/ml solution the within day precision was 5.7%. 4 The method has been applied to a patient receiving 300 mg/day viloxazine treatment.