An optimized capillary electrophoresis method for the analysis of doxycycline is described. The influence of methanol as organic modifier, buffer pH, buffer concentration, capillary length, column temperature, Triton X-100 and methyl-beta-cyclodextrin was systematically investigated. A central composite design was performed in order to optimize the method. The optimal separation conditions were: capillary, uncoated fused-silica [40 cm (32 cm effective length) x 50 microm I.D.]; background electrolyte, a solution of 145 mM sodium carbonate and 1 mM EDTA brought to pH 10.3-methanol (89:11, v/v); temperature, 15 degrees C; voltage, 12 kV. The method showed good selectivity, repeatability, linearity and sensitivity. Six commercial samples were quantitatively analyzed. The results were compared with those established by the liquid chromatography method from the European Pharmacopoeia.