A micro method for determination of indomethacin in plasma was developed. Following deproteinization of plasma with acetonitrile containing internal standard (mefenamic acid), the separation of indomethacin and internal standard was achieved by high-performance liquid chromatography using a 7 microm LiChrosorb-RP18 column (250x4 mm I.D.) at 50 degrees C. The mobile phase was 6 mM phosphoric acid-acetonitrile (50:50). The flow-rate was kept at 2.0 ml/min and the column effluent was monitored at 205 nm. The coefficients of variation of the method estimated at 0.2 and 1.0 microg/ml were 4.2 and 2.3%, and the detection limit of the drug was about 0.05 microg/ml (S/N=5). The method requires minimum pretreatment of the plasma with a small sample volume (25 microl), and is very suitable for therapeutic drug monitoring of indomethacin in premature infants with symptomatic patent ductus arteriosus.