Rob Lowe recalled duking it out with his pal and former co-star Tom Cruise when they were just teens on the set of the 1983 film “The Outsiders.”

During Monday’s episode of “The Rich Eisen Show,” the actor, 60, took a stroll down memory lane after host Eisen asked him to recall a story about working with the action star.

“He’s so competitive that we used to box in the hallway of the hotel we were staying during ‘Outsiders,’” Lowe recalled of Cruise. “So much testosterone. We’re 18-year-old guys stuck on location. So we would wear headgear and we’d have mouthpieces in, but we would legitimately spar.”

In the coming-of-age film, Lowe played compassionate teen Sodapop Curtis while Cruse, now 62, starred as cocky car lover Steve Randle.

Lowe said he wasn’t working out at the time and had “pipe cleaner” arms — and that Cruise was “jacked” in comparison.

“Tom is like, this beast,” he said. “But I hit him real clean and I rang his bell. And the next thing I knew, I woke up and I was coming to on the floor. He’d, like, completely knocked me out.”

As for what led the “Mission Impossible” star to TKO Lowe, the “Parks and Recreation” actor said it was because he had “hit him hard, like his eyes just went black.”

“But that’s the stuff we did, that’s what guys do, it’s like ‘Fight Club,’” he teased.

“The Outsiders” film, based on S. E. Hinton’s 1967 novel, also starred Patrick Swayze, Ralph Macchio, Matt Dillon, Emilio Estevez and C. Thomas Howell.

In a 2020 interview on the “Armchair Expert With Dax Shepard” podcast, the “West Wing” star shared another story about Cruise’s fiery spirit off-set.

According to Lowe, Cruise had flipped out when he learned that the two actors were sharing a room at The Plaza Hotel in New York City.

“First time I ever stayed at The Plaza Hotel. We check in and Tom finds out that we’re sharing a room, and just goes ballistic,” Lowe began his story.

“To me, what’s great about the story is: There’s certain people who have always been who they are, and that element of them has powered them to where they are today, and the rest is history,” he continued. “And the notion that an 18-year-old actor with a walk-on part in ‘Endless Love,’ and like, a seventh lead in ‘Taps,’ could have that kind of, like, wherewithal? I remember going, ‘Wow, this guy is the real deal.’”

Lowe said the moment was “gnarly” but made him “laugh.”

“But at the end, you can’t argue with the results. He’s had his eye on the ball since day one,” he added.

Watch a clip from Lowe’s interview on “The Rich Eisen Show” below.