index.php
<script type="text/javascript" src="jquery-1.4.2.js"></script>
<script type="text/javascript" src="ajax.js"></script>
<a href='one.php' class='ajax'>One</a>
<a href='two.php' class='ajax'>Two</a>
<div id="workspace">workspace</div>
one.php
$arr = array ( "workspace" => "One" );
echo json_encode( $arr );
two.php
$arr = array( 'workspace' => "Two" );
echo json_encode( $arr );
ajax.js
jQuery(document).ready(function(){
    jQuery('.ajax').live('click', function(event) {
        event.preventDefault();
        // load the href attribute of the link that was clicked
        jQuery.getJSON(this.href, function(snippets) {
            for(var id in snippets) {
                // updated to deal with any type of HTML
                jQuery('#' + id).html(snippets[id]);
            }
        });
    });
});
Above code is working perfectly. When I click link 'One' then String 'One' is loaded into workspace DIV and when I click link 'Two' then String 'Two' is loaded into workspace DIV.
Question:
Now I want to use a dropdown to load one.php and two.php in workspace DIV instead of links in index.php. When I use link then I use class='ajax' in link properties but how to call ajax request on drop down change event ?
Thanks