Behaviour.register({ '#SomeSelectElementsID': { onchange: function() { // Called when the element is changed if(this.value == 'the-value') { $('#TheElementToDisplay').style.display = ''; // Don't use 'block' as some browsers don't like it } else { $('#TheElementToDisplay').style.display = 'none'; // Hides the element } }, initialize: function() { // Called when the page is loaded if(this.value != 'the-value') { $('#TheElementToDisplay').style.display = 'none'; // Hides the element } } }});