slideDown won't work

Hi,

I have a div on the top of the site (msgbox) and at load, I hide this:

var btnRegisterClick=false;

$(document).ready(function () {
    if (btnRegisterClick == false) { $("#msgbox").hide(); }
    else { $("#msgbox").show(); }
});

If I click a button, I register this script:

function ShowMessageBox(type, text) {
    document.getElementById("msgbox").innerHTML = text;
    document.getElementById("msgbox").setAttribute("class", type);
    btnRegisterClick = true;
};

And the msgbox shows.

But if I change the show to slideDown, I don't get the effect, but the msgbox simply shows. I tryied to put the slideDown in the showmessagebox function, no success.

What can be the problem?