Permeability of the intestine to ⁵¹Cr-EDTA administered intragastrically was assessed in mice infected with Trichinella spiralis, in gerbils infected with Trichostrongylus colubriformis and in rabbits infected with Obeliscoides cuniculi. The results showed that, compared with the worm-free control animals, there was a significant increase in the recovery of ⁵¹Cr-EDTA in the urine five hours after the administration of the ⁵¹Cr-EDTA in all infected animals, about six times more in mice, ten times more in gerbils, and five times more in rabbits. Increased gut permeability appeared to be associated with changes in the mucosal surface of the gut, directly related to the action of the worms on the epithelium.