sliding menu - need animation?

hello all!  i have a client who is looking for a sliding menu in JavaScript.  I have a basic toggleMenu function written which works fine, however, they now want the menu to smoothly slide, not just appear.  does anyone know how to make the menu slide down as it appears?  below is my current easy toggleMenu function.

Thanks!  BTW, things are NOT absolutely positioned, so absolute positioning is a negative in this instance.

function toggleMenu(currMenu) {
                  if (document.getElementById) {
                        thisMenu = document.getElementById(currMenu).style
                        if (thisMenu.display == "block") {
                              thisMenu.display = "none"
                        }
                        else {
                              thisMenu.display = "block"
                        }
                        return false
                  }
                  else {
                        return true
                  }
}