<asp:DropDownList runat="server" ID="select1" name='select1' OnSelectedIndexChanged="select1_changed">
  <asp:ListItem value='1'>1</asp:ListItem>
  <asp:ListItem value='2'>2</asp:ListItem>
  <asp:ListItem value='3'>3</asp:ListItem>
</asp:DropDownList>

<div id='textboxContainer'>
  <asp:TextBox runat="server" ID="text1" name='text1'></asp:TextBox>
  <asp:TextBox runat="server" ID="text2" name='text2'></asp:TextBox>
</div>