// myfrm.form.txtCity.value = myfrm.options[myfrm.selectedIndex].value;
myfrm.form.txtCity.value = myfrm.options[myfrm.selectedIndex].text;
myfrm.form.txtCity.value = myfrm.options[myfrm.selectedIndex].text;
<option value="0">(请选择)</option>
<option value="1">长沙</option>
<option value="2">株洲</option>
<option value="3">湘潭</option>
</SELECT>
<INPUT type="text" name=txtCity >
<script language=JavaScript>
function onCityChange(myfrm)
{
if (myfrm.selectedIndex > 0 )
{
txtCity.value = myfrm.options[myfrm.selectedIndex].text;
myfrm.options[myfrm.selectedIndex]=null
}
}
</script>