The separation of two different sets of synthetic peptides has been investigated by high-performance capillary zone electrophoresis utilising naked, fused silica capillaries. The effects of electrolyte pH, buffer concentration, capillary length and electric field strength on the separation efficiency and selectivity were systematically varied, with the highest resolution achieved with buffer electrolytes of low pH and relatively high ionic strength. Under optimised separation conditions utilising the "short end injection" separation approach with negative electric field polarity, a series of eight structurally-related synthetic peptides were baseline resolved within 4 min without addition of any modifier of the background electrolyte with separation efficiencies in the vicinity of 600000 theoretical plates/m. Further significant enhancement of separation efficiencies could be achieved by taking advantage of the "long end injection" approach with positive electric field polarity. The outcome of these experimental variations parallels the "sweeping" effect that has been observed in the capillary electrochromatographic and micellar electrokinetic separations of polar molecules and permits rapid resolution of peptides with focusing effects. In addition, small changes in the electrolyte buffer pH and concentration were found to have a significant impact on the selectivity of synthetic peptides of similar intrinsic charge. These observations indicate that multi-modal separation mechanisms operated under these conditions with the unmodified fused silica capillaries. This study, moreover, documents additional examples of peptide-specific multi-zoning behaviour in the high-performance capillary zone electrophoretic separation of synthetic peptides.