A fully automated method is described, which enables the determination of indomethacin in plasma by reversed-phase HPLC following on-line sample enrichment and clean-up on a short pre-column. The plasma sample is introduced directly into the column switching system. The pre-column, filled with a pellicular bonded phase, is first washed with phosphate buffer, pH 7.4. The compounds retained on the pre-column are then eluted in the fore-flush mode and separated on an octadecylsilica column with a methanolic phosphate buffer (pH 7.4) mobile phase. Indomethacin is determined spectrophotometrically at 254 or 260 nm. The effect of changes in the pH and flow rate of the washing eluent are studied. Under the conditions selected, memory effects can be avoided, the absolute recovery of the drug is 70% and the limit of detection 10 ng ml-1 for a 100 microliter injection of plasma. At a concentration of 100 ng ml-1, the relative standard deviations (RSD) are 1.7% (within-day) and 3.5% (between-day), respectively.