I've tried this :
<div id="xDiv" align="left">
<div id="divX" runat="server" style="margin-left:40px; width: 650px;"
align="center">
<asp:Menu ID="Menu3" runat="server" Orientation="Horizontal" Width="100%"
StaticSelectedStyle-CssClass="StaticSelectedStyle" Height="52px"
StaticSubMenuIndent="18px" oninit="Menu3_Init">
<StaticSelectedStyle CssClass="StaticSelectedStyle"></StaticSelectedStyle>
<Items>
<asp:MenuItem Text="Test" />
</Items>
</asp:Menu>
</div>
<div runat="server">
<asp:DropDownList ID="DropDownList3" runat="server"
DataSourceID="SqlDataSource1" DataTextField="RepGroup_Name"
DataValueField="RepGroup_ID" Width="162px">
</asp:DropDownList>
</div>
</div>
but it puts DropDownList not to the end of line , it puts it on a new line.