After a long latency, hormone-producing islet cell tumors are induced with high frequency by a single administration of streptozotocin and nicotinamide in the rats. Thus, pancreatic islets must be considered a target tissue for the tumorigenic, as well as diabetogenic, action of streptozotocin. These tumors are well-differentiated and resemble normal islet tissue, both morphologically and functionally, as they are rich in typical beta cells and release insulin in response to glucose, both in vivo and in culture. However, some noteworthy differences have been observed in comparison to normal tissue: (a) the release of large amounts of proinsulin after glucose stimulation, (b) the presence of granulated cells of uncertain classification, similar but not identical, to delta cells, and (c) the altered proportion in hormone content, which suggests a different sensitivity of the different islet cell types to the chemically induced transformation.