Cation-selective exhaustive injection and sweeping followed by a MEKC separation is evaluated for the sensitive analysis of 5-nitroimidazoles in untreated human serum and urine. Deproteinized serum and urine samples were diluted 76 and 143 times, respectively, in a low-conductivity solvent (5.00 mM orthophosphoric acid containing 5.0% v/v methanol). Samples were electrokinetically injected at 9.8 kV for 632 s in a previously conditioned fused-silica capillary (65.0 cm x 50 mu m id). Separation was performed at -30 kV and 20 degrees C using 44 mM phosphate buffer (pH 2.5), 123 mM SDS, and 8% v/v tetrahydrofurane as BGE. Signals weremonitored at 276 nm and peak area was selected as analytical response. Good linearity (R-2 >= 0.988) and LODs lower than 1.5 and 1.8 mu g/mL were achieved in serum and urine, respectively.