Solid-phase extraction (SPE) and capillary electrophoresis (CE) are on-line coupled via a Tee-split interface, which provides hydrodynamic injection of the SPE eluate by flow splitting. The interface allows sample preconcentration independently from the CE separation and prevents sample matrix and washing solvents from entering the separation capillary. The effect of the Tee-split interface on the CE efficiency was examined using enkephalin peptides as model compounds. Most favorable plate numbers were obtained using a split ratio of 1:40. Breakthrough volume, desorption efficiency and elution volume for the C18 micro SPE column (5 mm x 0.5 mm i.d.) were found to be 750 microL, 65% and 1 microL, respectively. The performance of the complete system was demonstrated by the preconcentration and separation of an enkephalin mixture. Plate numbers up to 120,000 were obtained using a sample volume of 250 microL and a split ratio of 1:40. Enkephalin peak areas were linear (R2 = 0.996) over the 10-1000 ng/mL range. UV absorbance concentration limits of detection (SIN = 3) were about 5 ng/mL. For 250 microL injections of 100 ng/mL, the relative standard deviation (n = 5) of peak area was lower than 10%.