The storage of insulin and proinsulin in various subcellular compartments of an islet adenoma and islets from the adjacent pancreas was compared. The tumor had a greater concentration of proinsulin relative to insulin in the secretion granule fraction than the adjacent islets. The microsome fraction from both the adenoma and islets had a larger percentage of proinsulin compared to insulin in contrast to other fractions. In a tolbutamide tolerance test before surgery, the percentage of serum proinsulin relative to insulin was 53% before and 17% after tolbutamide. Proinsulin secretion was temporally separated from insulin secretion in the early phase.