Dynamic setting of data source

Hi, I have a query regading data source attribute of Item Dropdown field of a form. Is it possible to set the data source (Item Type) of a dropdown (field type as item) dynamically/programmatically? Can you let me know how I can achieve this? I would like dynamically set the attribute of a field - a dropdown box of field type 'item' to a particular Item Type based on different selections of the user. Can you please provide a way to achieve this dynamic setting of data source attribute of dropdown box of field type 'item'?   Alexis.