To overcome the deleterious effects of hyperglycemia on islet transplantation, we have used recombinant basic fibroblast growth factor (FGF), an angiogenic peptide, concurrently with the intrasplenic placement of neonatal islets in severely diabetic rats. In both experimental and control rats, a minipump, secured to the spleen with a small indwelling catheter, delivered either the basic FGF at 10 ng/hr, or only the diluent. The rate of cure in the animals receiving the islets plus the peptide was significantly higher (70%) than in control rats (20%). This difference (P less than 0.05), could not be attributed to direct effects of the peptide on islet function or to increased islet cell replication. We believe that these results may be best explained as an enhancement by basic FGF of the angiogenic process in the transplant islets.