Sir Peter Ustinov (Manny the Rat) and Andy Devine (The Frog) also did voice work for Disney's Robin Hood (1973), in which Ustinov voiced Prince John and King Richard, and Devine voiced Friar Tuck.
One of the minor toys who appeared at the beginning of the film and later in the climax and end scene (the toy duck with the sailor hat) bears resemblance in design to the Dirty Duck from co-director Charles Swenson's earlier Murakami-Wolf film of the same name, Dirty Duck (1974).
Author of the original book, Russell Hoban, deeply disliked how the film "butchered" the source material and how some of its philosophical themes were watered down. However, he did praise the voice work of Peter Ustinov (Manny) and Cloris Leachman (Euterpe).
During Manny's first scene after the mouse and his child find themselves in the dump, the words that briefly appear on the bottom of the father mouse when a tractor approaches and the rat grabs the two out of the way in a panic read "Made in Japan". The film is a co-production with Sanrio (a Japanese toy company).
Manny was given the full name "Emmanuel Wolfington Rat III" for the film adaptation, as he introduces himself to the mouse and his child in his first scene.