EXCLUSIVE

Playboy Playmate Cassandra Lynn was found dead in her friend's L.A. home Wednesday ... and police are investigating the death as a possible overdose, law enforcement sources tell TMZ.

We're told Lynn's friend found her floating in a bath tub. The friend then called police, reporting a possible OD. When paramedics arrived they could not revive her. Foul play is not suspected.

Lynn -- Miss February 2006 -- was 34.

Cassandra was pretty active on Twitter ... her last real tweet was three days ago.