The scene in which Fievel presses up against a window to look into a classroom filled with American "schoolmice" is based on a story Steven Spielberg remembered about his grandfather, who told him that Jews were only able to listen to school lessons through open windows while sitting outside in the snow.
"Somewhere Out There" became a major hit, peaking at #2 in the Billboard Hot 100, winning a Grammy award, and became one of the most popular songs from an animated feature since the 1950s.
Originally, the concept consisted of an all-animal world, like Disney's Robin Hood (1973), but Don Bluth suggested featuring an animal world existing as a hidden society from the human world, like Disney's The Rescuers (1977). After viewing the film, Steven Spielberg agreed. Don also worked on both these films during his time working at The Walt Disney Company.
Nehemiah Persoff was chosen to voice Papa Mousekewitz mostly because he had a similar role as Barbra Streisand's father in Yentl.