The on-line combination of capillary zone electrophoresis (CZE) with capillary isotachophoresis (ITP) increases significantly the separation capability and sensitivity of capillary electrophoresis. This technique was used for separation and quantification of fourteen selected natural constituents in red wine belonging to flavonoids and phenolic acids. The leading electrolyte (LE) in the ITP pre-separation step was 10 mM HCl of pH* 7.2 with Tris as counterion, the terminating electrolyte (TE) was 50 mM boric acid of pH* 8.2 (adjusted with barium hydroxide). The background electrolyte in the electrophoretic step contained 25 mM beta-hydroxy-4-morpholinopropanesulfonic acid (MOPSO), 50 mM Tris, 15 mM boric acid and 5 mM beta-cyclodextrin of pH* 8.5. The content of methanol in all electrolytes was 20% (v/v). For exact timing of the transfer of isotachophoretically stacked analyte zones into the CZE column and for the control of the residual amount of leading and terminating ITP electrolytes picric acid was used as coloured marker. The R.S.D. values (n = 6) ranged between approximately 0.1% (for 0.25 microg ml(-1) rutin) and approximately 11% (for 0.25 microg ml(-1) of quercitrin). Detection limits were 30 ng mi(-1) for phenolic acids, quercitrin and rutin, 100 ng ml(-1) for quercetin, kaempferol and epicatechin and 250 ng ml(-1) for catechin. A single analysis took 45 min.