Jennifer Aniston is "totally into" Justin Theroux.

The former 'Friends' star - who was previously married to Brad Pitt and has dated Vince Vaughn and John Mayer - is said to be smitten with the 'Your Highness' actor and can't stand being apart from him.

A friend of the 42-year-old star - who first met Justin on the set of movie 'Wonderlust' - told Us Weekly magazine: "Jen is totally into him. They have been hanging out for months.

"She wants to be with him all the time. She's diving in headfirst."

Earlier this week, it was reported Jennifer and Justin's relationship is getting serious after the pair spent the entire weekend together although they are not ready to go public with their romance.

A source said: "These two are jumping into this head-first. They're together all the time. But they sneak around and do everything they can not to be photographed together. Jen doesn't like her relationships to go public before she's ready."

While Justin visited Jennifer at her home last Friday (20.05.11), the couple had a romantic lunch at the Beverly Hills hotel on Saturday (21.05.11) before leaving together.

Jennifer then threw a small party at her house on Sunday (22.05.11) with the actor and a few other close friends in attendance.