Indomethacin is an inhibitor of prostaglandin synthesis and has several uses, including the ability to encourage closure of a patent ductus in premature neonates. A rapid, accurate, and sensitive high-performance liquid chromatographic (HPLC) method is described for measuring plasma concentrations of indomethacin. An acidic buffer (citrate, pH 3.0) was employed to alter the pH of the aqueous phase prior to extraction to minimise interference from endogenous compounds. Extraction of indomethacin from plasma was optimally achieved with petroleum ether (boiling fraction of 40–60°C):dichloromethane (50:50). However, separation of indomethacin from plasma proteins was attempted unsuccessfully using acetonitrile precipitation; severe band broadening occurred due to injected sample solvent problems. The absolute recoveries for indomethacin and internal standard (mefenamic acid) were over 90% (n = 3). Precision was expressed as the coefficient of variation (n = 4), which was ±8% at each of six indomethacin concentrations in the range 50–2,000 ng/ml. The limit of quantitation of indomethacin in plasma was 50 ng/ml (0.5 ml of plasma injected). We report an HPLC method for the quantitation of indomethacin in plasma that may have widespread applicability for routine drug assay laboratories.