In the present study we have investigated whether the pancreatic B-cell toxic agent streptozotocin (SZ) induces increased vascular permeability as an indicator of lesions in the pancreatic islets. SZ was given either in multiple low doses, providing an animal model for Type I diabetes mellitus with signs of autoimmunity, or by a single diabetogenic dose to C57BL/KsJ mice. The vascular reactions were detected by administration of Monastral blue B and the pancreatic islets were visualized by a freeze-thawing technique which made it possible to count the number of stained and unstained islets. This proved to be a rapid and sensitive technique for detection of early lesions within the islets after administration of SZ. It was found that the islets showed an increased vascular staining before the animals had become diabetic by either mode of SZ treatment and also before signs of pancreatic insulitis were found after the multiple low-dose injections of SZ. It is suggested that both types of SZ administration induce a B-cytotoxic reaction, essential for the development of hyperglycaemia. This leads to an activation of cells dealing with the disposal of cell debris, a process which probably also involves the release of substances mediating increased vascular permeability.