The disappearance of endogenous serum proinsulin and insulin was measured in 3 patients following removal of beta-cell adenomas of the pancreas. The mean half-disappearance time for proinsulin (17.2 min) was much longer than for insulin (4.8 min). This difference in the metabolic rate of the 2 hormones together with previously described differences in their hepatic extraction, are sufficient to explain the higher proinsulin :insulin ratio in serum compared to the pancreas.