Originally released with a "PG" rating in 1977, for some reason the studio wanted an "R" rating so they told the MPAA to re-rate the film as such in 1978.
A clip of this film can be seen in the fifth (and final) Dirty Harry film Scommessa con la morte (1988).
According to a contemporary article in 'The Los Angeles Times', animal trainer Karl Lewis Miller kept the dogs used in this film on a special diet of dry dog food and vitamins for several months to make them appear thin and hungry. Thirty-five dogs were trained for this film to portray the 15 dogs in the pack. Each of the dogs in the film had a body double. Josh, the lead dog, had been an animal shelter rescue for years before appearing in this film.
Charlton Heston was offered the lead role in the film, but turned it down.
Jerry's truck in the film is a 1975 International Harvester Scout II.