Enable/Disable a dropdownbox in jquery

Here is one way that I hope is easy to understand:

http://jsfiddle.net/tft4t/

$(document).ready(function() {
 $("#chkdwn2").click(function() {
   if ($(this).is(":checked")) {
      $("#dropdown").prop("disabled", true);
   } else {
      $("#dropdown").prop("disabled", false);  
   }
 });
});

I am using JQuery > 1.8 and this works for me...

$('#dropDownId').attr('disabled', true);

Try -

$('#chkdwn2').change(function(){
    if($(this).is(':checked'))
        $('#dropdown').removeAttr('disabled');
    else
        $('#dropdown').attr("disabled","disabled");
})

$("#chkdwn2").change(function(){
       $("#dropdown").slideToggle();
});

JsFiddle

To enable/disable -

$("#chkdwn2").change(function() { 
    if (this.checked) $("#dropdown").prop("disabled",true);
    else $("#dropdown").prop("disabled",false);
}) 

Demo - http://jsfiddle.net/tTX6E/