function dropdown_menu() {
 $("#sub-menu-id a, .subnav a");
 $(" #sub-menu-id ").css({
 display: "none",
 opacity: "1"
 }); // fix for opera browser
 $("#sub-menu-id li").each(function() {
 var $subNav = jQuery(this).find('ul:first');
 $(this).hover(function() {
 $subNav.stop().css({
 overflow: "hidden",
 height: "auto",
 display: "none"
 }).slideDown(350, function() {
 $(this).css({
 overflow: "visible",
 height: "auto"
 });
 });
 }, function() {
 $subNav.stop().slideUp(350, function() {
 $(this).css({
 overflow: "hidden",
 display: "none"
 });
 });
 });
 });
 }
 
 
 //USAGE 
 <script>
 jQuery(document).ready(function(){ dropdown_menu(); });
 </script>