Programmatically manipulate content type select list I have a content type called Car which have several field two of them are select lists which I want to make some operations on them. When the user select a value form the first select list a bulk of values should be populated in the other select list, my question is WHERE to do this programmatically and some tips for HOW doing this? I hope my question is clear

best
You need to use hook_form_alter to add AHAH/AJAX to your field. The hook should be placed in either a custom module or in your theme's settings.php file.

Drupal has many examples about this. Here's for Drupal 6 and here's for Drupal 7.

This specific example from the links above seems to be what you are looking for.