A fast and precise analysis of the synthetic peptide buserelin in urine using CZE-ESI-MS method has been demonstrated. Formic acid at 50 mmol/L concentration served as backgroud electrolyte in CZE stage and it is compatible with MS detection in positive ionization mode. Two injection modes were tested, i.e. pressure (50 mbar for 5 s) and electrokinetic injection (5 kV for 5 s), of which electrokinetic injection provided better calibration parameters. Buserelin LODs were 0 47 mu g/mL in water and 0 63 mu g/mL in ten times diluted urine samples using pressure injection, while they were 0.32 mu g/mL in water and 0.34 mu g/mL in ten times diluted urine samples using electrokinetic injection Repeatability of buserelin migration times was below 6% (pressure injection mode) and 1% (electrokinetic injection mode) Repeatability of buserelin peak area in SIM mode (m/z = 620.5 +/- 0.5) was less than 12% (pressure injection mode) and 5 8% (electrokinetic injection mode). In this work, no interferences were observed during the analyses of spiked urine samples.