Streptozocin-induced diabetic nude mice (blood glucose 493 +/- 14 mg/dl) received aliquots of 2000 human islet equivalents (IE) under the kidney capsule and were then followed for up to 30 days with measurement of blood glucose concentration and body weight. Characterization of islet aliquots before the implantation included the assessment of the endocrine beta cell and nonbeta cell mass, estimated by point counting morphometry of immunostained sections. Islet transplantation was followed by a rapid decrease in blood glucose levels and by a progressive increase in body weight; 15 days after transplantation mean glycemic levels were 102 +/- 11 mg/dl and further decreased to 70 +/- 3 mg/dl at 30 days. Despite the progressive improvement in the glucose levels, the beta cell mass of the islet grafts significantly decreased over time from 2.63 +/- 0.2 mg, at the time of transplantation, to 1.16 +/- 0.1 and 0.86 +/- 0.1 mg 15 and 30 days, respectively, after transplantation. In contrast, the endocrine nonbeta cell mass remained stable from before the implantation to 30 days after. Therefore, the endocrine nonbeta cell/beta cell ratio increased from 14% at the time of transplantation, to 35% and 37%, 15 and 30 days, respectively, after transplantation. The rate of replication of the transplanted beta cells was similar in the grafts harvested at 15 and 30 days, with the percentage of beta cells positive for bromo-2' deoxyuridine (BrdU) incorporation being in the range of approximately 0.1% 6 hr after the BrdU injection. These data demonstrate that an important decrease in beta cell mass takes place immediately after islet transplantation--the most dramatic decrease occurring in the first 15 days and persisting even after revascularization has occurred. However, endocrine nonbeta cell mass remained stable indicating that engrafted nonbeta cells are less likely to die than beta cells. The finding that the nonbeta/beta cell ratio of a human islet graft can increase over time, raises questions about whether such a change in islet structure could have an influence upon function.