In the present investigation two modifications of the injection technique (flow and batch) for an Hg ion-selective electrode have been applied. The optimal analytical parameters are found: sample volume, carrier flow rate, cell volume, tip-electrode distance, stirring rate. A limit of detection of 3×10^-7 mol 1^-1 for a sample of mercury perchlorate of about 0.5 ml is obtained for the flow-injection analysis.