By pulse labeling isolated rat islets with tritiated leucine, it was possible to follow the synthesis of proinsulin and the subsequent conversion of the proinsulin into an intermediate product and insulin. Examination of the islets fractionated at various times indicated synthesis of proinsulin in the microsomal fraction and transfer of the proinsulin to the secretion was the most important fraction in converting proinsulin to the intermediate product and insulin. The developing secretion granules are implicated in this conversion process. During incubation, insulin was secreted in a bimodal fashion. Insulin precursors were also released into the incubation media.