dropdownlist problem

Hi,
I just want to run a function when an item selected on dropdownlist. But it doesnt work. My codes below;
 in default.aspx page
-----------------------
 

    onselectedindexchanged="DropDownList1_SelectedIndexChanged"  >
 

-----------------------

in default.aspx.cs file
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  protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
  {
  Label1.Text = "Test";
  }
----------------------

I just want to write a test mesaage to lable1 but it isnt work. I mean "DropDownList1_SelectedIndexChanged" function doesnt run when I change the dropdownlist's item.

How can I run a function when I change the dropdownlist's item?

Regards.