Now, this is heavy! Michael J. Fox posted a pic with his beloved "Back to the Future" costar, Christopher Lloyd, sharing a reunion more than 40 years in the making.

That's right ... it's been more than 40 years since Doc Brown sent Marty McFly back in time at 88 mph, and the two are still great friends.

Fox wrote, "Dining with my bestie at the beach. Next year BTTF is 41. Great Scott. Chris will be 8️⃣8️⃣. That’s some serious s**t."

The dinner comes before the film's 41-year anniversary and after the publishing of Fox's memoir, "Future Boy," in which he recounts making the classic sci-fi.

Needless to say, the meeting was electric ... probably measured 1.21 gigawatts!