Can´t get dataGrid.select to work properly

All, I want to highlight several rows at the same time in a datagrid, as the user chooses with his mousebutton. I am using the following lines: System.Windows.Forms.DataGrid.HitTestInfo hti; hti = this.dataGrid1.HitTest(e.X,e.Y); this.dataGrid.Select(hti.Row); This works fine as long as the user doesn´t use his left mouseButton cause then everything that is highlighted disapears? Why doesn´t dataGrid.Select work when one is using the left mousebutton? I´m using the event dataGrid_MouseDown. Why is that? Thanks