We have compared the efficiency of foetal islet graft growth and function in renal subcapsular (systemic drainage) and splenic (portal drainage) sites. Age-matched female BALB/c streptozotocin (STZ)-diabetic mice were grafted either under the left renal capsule or on to the hilar surface of the spleen with one half of a syngeneic 17-day gestation foetal pancreas which had been in organ culture for 2 weeks. Gain in body weight, return to normoglycaemia and to normal glycosylated haemoglobin levels were not significantly different between the two groups. However, when an intraperitoneal arginine challenge was performed the portally grafted mice showed more normal responses than the systemically grafted mice. Although still not normal after the arginine challenge, mean blood glucose values were lower and serum insulin values higher in the spleen grafted animals than in the renal subcapsular graft group. The total amount of insulin was assayed by extraction of both the graft and the pancreas of each animal. The amount of insulin in the grafts from the spleen was not significantly different from those in the renal subcapsular site. We conclude that the efficiency of a graft draining into the portal circulation is greater than an equivalent sized graft draining systemically when a maximal challenge is applied.