Although the actresses have been best friends through thick and thin since the 1994 debut of, er, "Friends" on NBC, they haven't been photographed together in nearly a year.
"There's tension" between Aniston, 42, and Cox, 46, one source confirms in the new Us Weekly, out now.
What could possibly threaten such a close-knit friendship? Cox's trial separation from hubby David Arquette as she hangs with sexy "Cougar Town" costars Brian Van Holt and Josh Hopkins.
"Jen thinks Courteney is acting callous by running around with this guy but refusing to file divorce papers," an Aniston source says. "Like she's playing mind games with David." (Cox and Arquette announced their trial split in September 2010 after 11 years of marriage.)
Cox's post-separation antics with her costar even reminded Aniston of her own marital heartache with ex Brad Pitt, who took up with Angelina Jolie weeks after their 2005 split.
"She hates watching Courteney do the same thing," the source says.