Ajax jquery success scope

I have this ajax call to a doop.php.

    function doop(){
        var old = $(this).siblings('.old').html();
        var new = $(this).siblings('.new').val();

        $.ajax({
            url: 'doop.php',
            type: 'POST',
            data: 'before=' + old + '&after=' + new,
            success: function(resp) {
                if(resp == 1) {
                    $(this).siblings('.old').html(new);
                }
            }
        });

        return false;
    }

My problem is that the $(this).siblings('.old').html(new); line isn't doing what it's supposed to do.

thanks.. all helpful comments/answers are voted up.

Update: it appears that half of the problem was the scope (thanks for the answers that helped me clarify that), but the other half is that I'm trying to use ajax in a synchronous manner. I've created a new post