A sensitive and chemically specific high-speed liquid chromatographic method was developed for the determination of 4-nitro 2-phenoxymethanesulfonanilide in plasma. The method includes selective extraction of the anti-inflammatory drug and an internal standard, 2-(4'-chlorophenoxy)-4-nitromethanesulfonanilide, into benzene from acidified plasma followed by reextraction into 0.2 N NaOH. The aqueous layer is acidified, and the drug is reextracted into benzene. The benzene is evaporated, and the residue is dissolved in a small volume of acetonitrile. A 10-microliter aliquot is analyzed on a reversed-phase column. The mean overall extraction recovery, after correction for aliquot factors, is 99%. The accuracy, expressed as the relative error, is 4, 0.3, and -3% at 0.60, 1.50, and 3.00 microgram/ml, respectively. Repeated analysis of reference standards indicates that the precision, expressed as the relative standard deviation, is 3% or less. The lower sensitivity limit is 0.2 microgram/ml with a 2-ml plasma sample. The method was applied successfully to the determination of plasma levels of 4-nitro-2-phenoxymethanesulfonanilide in humans and rats in metabolic experiments at pharmacological doses.