How To Hide An Option In A SELECT Tag

I need to hide/show elements in a SELECT tag. I don't want to manage 2+ SELECTs (populating one from another) I'm trying to do something like this:

function actualizar_lista_platos(){
var lp = document.getElementById('COD_PLATO');
var mnu = document.getElementById('COD_MENU');
var cod_menu = mnu.value;

[Code]...

Both COD_MENU and COD_PLATO are SELECTs, being COD_PLATO the "son" of COD_MENU (every COD_MENU element has associated 1+ COD_PLATO's element, so if I choose a COD_MENU element, I need in COD_PLATO to be shown the COD_PLATO's elements related to the select COD_MENU item)

Finally, I just need to know if there's any way to show/hide elements in a SELECT tag. That's it.