try<select id="selist" style="display:none">
<option value="1">1</option>
<option value="2">2</option>
</select>
<input type="text" id="txt"><br>
<input type="button" id="btn" value="show list" onclick="showItem()">
<script language="javascript">
function showItem()
{
  if (selist.style.display=="none")
  {
selist.style.display="";
txt.style.display="none";
btn.value="show text";
  }
  else
  {
selist.style.display="none";
txt.style.display="";
btn.value="show list";
  }
}
</script>