Ron Howard plays a boy genius who invents a super growth formula. He later played the same kind of role in Genius at Work
Vicki London was originally cast with a larger role, but declined to disrobe in preproduction, so her part was changed with Joy Harmon's, and much of London's subsequent screen time was cut. Her agent kept her in the movie at full pay, while London didn't seem to object to what happened, since she saw the whole thing as forgettable.
An alternate version of the theme music - "The Last Race" - was reused by Quentin Tarantino in Death Proof.
Exteriors were shot on the Columbia Studios backlot (now part of the Warner Bros. Backlot), the same lot as the exteriors for the TV series Bewitched and I Dream of Jeannie. Many scenes were shot on Courthouse Square at Universal Studios, which doubled as Hill Valley in Back to the Future.
Filmed in "Perceptovision".
In one scene one of the giants reads an issue of "Famous Monsters of Filmland" with another Bert I. Gordon film, War of the Colossal Beast, in the cover.
Loosely based on the H.G. Wells story "The Food of the Gods", about a substance that causes giant mutations in growing organisms. Children fed the substance become giants (capable of producing giant offspring), who choose to fight when their existence is threatened by adult authorities.
The "Teen Magazine" that Merrie (Joy Harmon) reads was an actual issue of the magazine published in July 1965.
The beer that the delinquent teens drink after crashing their car is the once popular Blatz Beer.
The brand of chicken that the giant teenagers eat is a tie-in to the once-famous restaurant chain called Chicken Delight. The chain was known for home delivery of chicken and ribs, as well as it's catchy motto: "Don't cook tonight, call Chicken Delight." A banner for the restaurant chain can be seen on a wall behind the adults who turn in their rifles.