The clinical and pathologic features of a malignant schwannoma of the lumbosacral plexus after streptozotocin (STZ) therapy in a substrain of Wistar rat are described. The lesion occurred in one of a cohort of 40 rodents in which diabetes had been induced through the use of STZ; these animals were being prepared for fetal rat pancreatic transplantation studies with the ultimate objective of reversing the hyperglycemic state. The abnormalty became clinically apparant 9 months after the administration of STZ, a potential carcinogen, by the presence of an abdominal mass and paralysis of a back leg. Immunohistochemistry revealed that the character of the growth was compatible with that of a malignant peripheral nerve sheath tumor.