allow readers to see some posts, users to see all?

Until now my blog has been password-protected. I’d like to remove the password-protection so that the blog is visible to all, but keep some posts visible only to those who are logged in – is there a way to do this? Passwording individual posts doesn’t accomplish what I want – I’d like all posts to be visible as soon as someone has logged in. I suspect this has something to do with user levels, but can’t figure out how to do this. TIA