<select id="selectID" onchange="return showMessage()">
     <option value="0">==請選擇==</option>
     <option value="1">是</option>
     <option value="2">否</option>
</select>

<script type="text/javascript" language="javascript">
  function showMessage() {
      if (document.getElementById("selectID").options[document.getElementById("selectID").selectedIndex].value == 1) {
   alert("你好，你選擇了第 1 個");
   document.getElementById("txtContractName").setAttribute("enable",false);
      }
      else if (document.getElementById("selectID").options[document.getElementById("selectID").selectedIndex].value == 2) {
   alert("你好，你選擇了第 2 個");
      }
  }
</script>

//js操作服務器控件dropdownlist
<asp:DropDownList ID="ddlFolder" runat="server" SkinID="ddlSkin" AutoPostBack="false" OnSelectedIndexChanged="ddlFolder_SelectedIndexChanged">   
        <asp:ListItem Value="0">選項0</asp:ListItem>  
   <asp:ListItem Value="1">選項1</asp:ListItem> 
</asp:DropDownList>   
<asp:DropDownList ID="ddlFolder" runat="server" SkinID="ddlSkin" AutoPostBack="false" OnSelectedIndexChanged="ddlFolder_SelectedIndexChanged">
  <asp:ListItem Value="0">選項0</asp:ListItem>
  <asp:ListItem Value="1">選項1</asp:ListItem>
</asp:DropDownList>

document.getElementById("ddlFolder").value="0";//0為你要選中的項的value

var DropDownListCurrencyNew =  document.getElementById("ddlFolder");
for(i = 0; i < DropDownListCurrencyNew.options.length; i++)
{
          if(DropDownListCurrencyNew.options[i].text == "選項0")   
           {
               DropDownListCurrencyNew.options[i].selected = true;
          }
}

var selValue = document.getElementById("DropDownList1").options[document.getElementById("DropDownList1").selectedIndex].value;

var selText = document.getElementById("DropDownList1").options[document.getElementById("DropDownList1").selectedIndex].text;