An earlier developed capillary electrophoresis, (CE) system with an on-capillary adsorptive phase is investigated for its suitability to quantitate low concentrations of angioten-sin II and gonadorelin in plasma. An off-line solid-phase extraction is used for sample preparation. The on-line preconcentration CE system allows multiple capillary volumes of sample solution to be injected, increasing the concentration sensitivity of CE with 3-4 orders of magnitude. Furthermore, possible influence of matrix salts can be ruled out by employing a rinsing step after sample application. Using short-wavelength UV detection, reproducibility and linearity in the low nanomolar range were satisfactory. The capillary could be efficiently-regenerated using a programmed between-run rinsing procedure, allowing 20-30 large injections of sample extracts. Coating of the capillary, improved the robustness of the method. Mass spectrometric detection via a previously reported sheathless interface increased the selectivity and sensitivity substantially. Recommendations are provided for the sample preparation process, the most critical part of the system. Further purification of the sample is required to allow the loading of larger sample volumes and to optimize the system's robustness.