script broken when change to 1.7 version

script broken when change to 1.7 version

HI friends! I need your help... when I change to jquery new version (1.7), my menu script was broken, could anybody help me to fix it? thanks!!
  1.     function mainmenu(){
    // Oculto los submenus
    $("#nav ul").css({display: "none"});
    // Defino que submenus deben estar visibles cuando se pasa el mouse por encima
    $(" #nav li").hover(function(){
        $(this).find('ul:first:hidden').css({visibility: "visible",display: "none"}).slideDown(400);
        },function(){
            $(this).find('ul:first').slideUp(400);
        });
    }