Histopathological examination of heart, lungs, liver, spleen, kidney, pancreas and gastrointestinal tract revealed no changes except in liver and lungs (Table 7). The various changes observed in the liver and lungs (except parasitic cysts in liver) are features commonly seen in all colony-bred animals and as such do not have any great signi®cance. Hence it was concluded that PSO did not cause any toxic changes in the tissues examined.