A rapid fully automated method for the determination of amino acids is described based on high-performance liquid chromatography and pre-column o-phthaldialdehyde derivatization. Using a 150 mm x 4.6 mm I.D. HPLC column filled with a recently introduced 2-3 microns Spherisorb ODS II packing material, 30 physiological amino acids could be determined within 28 min (injection to injection), while 95 samples could be processed unattended within 45 h. For most amino acids, the coefficient of variation (C.V.) for the peak areas measured was below 3%, both in aqueous standards and in plasma. Providing a pre-column change every 200-300 runs, the separation remained unaltered for about 1500-2000 runs on a single column.