To add AJAX to existing node (content type) in Drupal 7 Every Add/Edit node has a form_id. We can use that form_id to alter the form to AJAX handler to any form element. To find form_id of node use Drupal's devel module (dpr() function) or simply just write print_r(form_id) in hook_form_alter() function. To add AJAX handler to node form field write the following code in the hook_form_alter(&$form, &$form_state, $form_id) $form['field_make'][LANGUAGE_NONE]['#ajax'] = array( 'wrapper' => 'trailer-model', 'callback' => 'smr_manage_trailer_populate_models_callback', 'method' => 'replace', 'progress' => array( 'type' => 'throbber', 'message' => "loading", ), ); Here field_make is node form field in which we are adding AJAX. Adding AJAX is same as we do in other ...