An attempt was made in the early 90's to re-release 'Underdog' in syndication but because of the drug culture at the time all references to the "secret energy pill" were removed. Without them the cartoons did not make sense and were quickly pulled. This is a little ironic since the original reason the pills were added was to encourage children to take their vitamins.
The episode "Simon Says, 'No Thanksgiving'!" is unique in that it marks the one time that Underdog does not speak in a rhyme when he says "Not plane nor bird nor even frog. It's not even little old me, Underdog. It's a balloon."
The gangster Riff Raff's voice is an imitation of that of the gangster movie star George Raft by voice actor Allen Swift.
When the series was brought to Japan in 1967, it was called "Urutora Wan-chan" ("Ultra-Puppy"), due to the huge popularity of Ultraman: A Special Effects Fantasy Series (1966) and Ultraseven (1967) at the time, and because "Ultra" cleverly fit the "U" on Underdog's chest.