Endocrine B cells differentiate normally in embryonic rat pancreatic rudiments cultured in vitro. The specific concentration of immunoreactive insulin based on total protein increases by about 1000-fold during the developmental period, corresponding to days 13--20 of gestation. The rate of (pro)insulin synthesis, measured from the level of radioactive leucine incorporated into insulin, quantitatively accounts for the insulin accumulated during this period. In addition, the relative incorporation of leucine into proinsulin compared to insulin is constant during development and is similar to that found in the B cells of adult islets. Thus, there appears to be no significant change in the rate of conversion of proinsulin to insulin during B cell differentiation.