Apache :: Internal URL Rewrite Across Subdomains

I want to rewrite across subdomains, e.g. requests for sub1.mydomain.com/foo.htm -> www.mydomain.com/foo.htm. I can do

this with something like:

Code:
RewriteCond %{HTTP_HOST} ^sub[0-9].
RewriteRule (.*) http://www.mydomain.com/$1 [R=303,L]

I want to make it an internal rewrite though, not an external redirect. If I remove the [R=303] I still end up with a redirect (a 302). I assume mod_rewrite is forcing an external redirect because it's to a different [sub]domain.

Is there any way I can overcome this and make the change internal?