problem in applying javascript ??????

Hi,
     i have a dropdownlist to which i want to apply some javascript on the click of a button.
     I am doing it as follows on the click event of Button:
        dropdownlist1.Attributes.Add( "onchange" ,  "javascript:return FillCombo()" ); 
The code of javascript function is as follows :
        function FillCombo()
        {
            alert('HI');
        }

 But its not working as intended.

What's the problem ??
 
Thanx in advance
 
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