programmatically set value in dropdown list resides in nested content or archetype

Hi. I have a dropdown list consists of 2 values. When I create a new content (using the content service) I want to set one of the 2 available values in the list. So far nothing helps, it enters the DB with no value at all. I can set the rest of the nested content properties just fine (textstrings). I tried the datatype service after saving the content, but no luck there either. Sadly, same goes for the archetype model. Can anyone help?

thanks, Zohar.