The "Friends" and "Horrible Bosses" star was spotted going into the hospital with her new boyfriend, actor-screenwriter Justin Theroux to visit Dow Friday.
When Dow suffered the stroke and what her current condition is were not reported.
"Justin has been very supportive and there with [Aniston] all the way," an unidentified source told UsMagazine.com. "It's a very hard time for her right now."
Aniston's publicist declined to discuss the story, telling MSNBC, "This is a private family matter."
The actress and her mom were on the outs for years after Dow wrote the 1996 book "From Mother and Daughter to Friends: A Memoir," however, they had since reconciled, UsMagazine.com said.