AbstractA sensitive and specific method for the estimation of theophylline−7‐acetic acid in plasma by high performance liquid chromatography is described. Acidified plasma is extracted with chloroform‐n‐butanol (85: 15), back extracted with phosphate buffer (0.5 moll−1, pH 6.5), and finally the acidified aqueous phase extracted again with chloroform‐butanol. After evaporation of the extract the residue is redissolved in the chromatographic mobile phase (hexane‐dichloromethane‐ethanol‐acetic acid, 10:86: 3:1) and chromatographed on silica gel. The method was used to follow the plasma theophylline‐7‐acetic acid concentrations in two volunteers after single oral doses of 1.0 g of the drug. The results confirm previous reports, based on urine data and on plasma data following intravenous administration, that the drug is poorly absorbed (apparent bioavailability of 1 and 2 per cent), rapidly eliminated (half‐life of 1.0 and 2.4 h) and not co