[html]
<select id="mysel" name="mysel">
   <option value="1">1 xxxxxxxxxx</option>
   <option value="2">2 yyyyyyyyyy</option>
   <option value="3">3 zzzzzzzzzz</option>
   <option value="4">4 wwwwwwwwww</option>
</select>
<button onclick="setSel(3)">设置第3项为选中项</button>
<button onclick="clearSel()">清空选择框</button>
<button onclick="fillSel()">填充选择框</button>
<button onclick="removeSel()">移除第一项</button>
<button onclick="editSel()">修改第一项</button>

<script type="text/javascript">
     function setSel(str){
         with(document.all){    
             for(var i=0;i<mysel.options.length;i++){                
                 if (mysel.options[i].value==str){
                     mysel.selectedIndex=i;
                     break;
                 }
             }            
         }
     }

     function clearSel(){
         with(document.all){    
             mysel.options.length=0;
         }
     }

     function fillSel(){
         with(document.all){
             mysel.options.length=0;
             mysel.options[0] = new Option("1 xxxxxxxxxx","1");
             mysel.options[1] = new Option("2 yyyyyyyyyy","2");
             mysel.options[2] = new Option("3 zzzzzzzzzz","3");
             mysel.options[3] = new Option("4 wwwwwwwwww","4");
             mysel.options[4] = new Option("5 aaaaaaaaaa","5");

             mysel.selectedIndex = 4;
         }
     }

     function removeSel(){
         with(document.all){
             mysel.remove(0);
             if (mysel.options.length>0){
                 mysel.selectedIndex=0;
             }
         }
     }

     function editSel(){
         with(document.all){
             if (mysel.options.length>0){
                 mysel.options[0] = new Option("这是新的第一项","new Value")
                 mysel.selectedIndex=0;
             }
         }
     }
</script>

[/html]