Question

why slideDown() and slideUp() does not work true on my menu?
They are repeated several times with mouseover().

see you: jsfiddle-my codes

$('#nav li').mouseover(function() {
        $('#nav li ul li').slideDown();
    })
    .mouseout(function(){
        $('#nav li ul li').slideUp();
    })

Answer 1

I would do this:

http://jsfiddle.net/5KEnp/4/

$('#nav > li').mouseenter(function() {
    $('ul li', this).stop(true, true).slideDown();
}).mouseleave(function(){
    $('ul li', this).stop(true, true).slideUp();
})