The dogs had difficulty seeing in the rat costumes, so meat and blood was used to get them to run in the right direction in any given scene.
James Herbert, who wrote the novel upon which the film was based, was displeased with what the filmmakers did to his novel.
The giant rats, in full shots, were actually Dachshunds dressed in rat suits. Detailed puppets were used in close shots (especially in the scenes of their gnawing on victims).
Thirty-five dachshunds and five terriers were used to play the mutant rodents in this film.
In 1988 James Herbert, author of the source novel, dismissed this and the earlier The Survivor (1981) as "terrible ... absolute rubbish."