Studies were conducted to examine the insulin and proinsulin synthetic response to glucose in the streptozotocin-nicotinamide-induced rat islet adenoma. Tumors responded to an increase from 3.8 mmol/L to 16.6 mmol/L glucose by increasing incorporation of [3H]-L-leucine into both proinsulin and insulin. Though this response was statistically significant, the stimulation was less than that noted in rat islets, and the variability of incorporation was greater. In addition, the conversion of proinsulin to insulin was generally slow, again with substantial intertumor variation. The recovery of insulin, as well as proinsulin, was significantly higher at the end of four hours of incubation in tumors incubated in 16.6 mmol/L glucose, implicating an insulin-degradative pathway modulated by glucose. Therefore, while the tumor will not replace conventional sources of tissue for insulin biosynthetic experiments, systems utilizing the tumor can serve as an addition to the methodology for studying previously unrecognized or poorly understood intracellular processes within the beta cell.