Rat pancreatic islets were exposed to a combination of analogs of arginine (3 mM canavanine) and lysine (3 mM thialysine) for 2 h and then labeled with [3H]leucine in the continued presence of the analogs. Control islets were incubated in parallel without analogs. Prelabeled islets were then incubated for a 3-h chase period without analogs. Incorporation of the analogs blocked conversion of newly synthesized, radioactive, proinsulin to insulin. No untoward toxic effects of the analogs were found on nonradioactive insulin as measured by radioimmunoassay. The rate of release of the modified proinsulin was no different from that found for proinsulin newly synthesized in the absence of analogs and, as such, susceptible to the normal action of the enzymes responsible for proinsulin to insulin conversion. These results confirm the previously untested hypothesis that the beta-granule is the minimal functional unit of release; granules are thus handled by the B-cell and released at a rate which is independent of the physicochemical nature of their contents.