Rob Reiner & Son Nick in Heated Argument at Conan O'Brien's Christmas Party

Rob Reiner, his wife Michele, and their son Nick attended Conan O'Brien's Christmas party Saturday night. At the party, Rob and Nick got into a "very loud argument," loud enough for many others to hear ... Reiner family sources tell TMZ. We're told Rob and Michele then left the party. We do not know if Nick left as well.

TMZ is the first to report the argument between Rob and Nick before the murders.

Our family sources add … Michele had been anguishing to friends over the last few months that she and Rob were at their wits' end over Nick's mental illness and alleged substance abuse issues, and did not know what to do with their son Nick … saying, "We've tried everything."

As you know ... the legendary actor-director and his wife were found in their L.A.-area house Sunday with their throats slit. Nick was arrested Monday morning and booked for murder ... he's held on $4 million bail.