The effect of hyperglycaemia on islet transplantation in the rat has been examined in two ways, using syngeneic transplantation of 400 islets to the kidney capsule and subsequent measurement of kidney insulin content as a measurement of B-cell survival. Firstly, islets were transplanted into either diabetic or normal rats, then 6 months later the composite kidney/islet graft was transplanted into a normal rat. The insulin content was measured after a further 6 months and was found to be significantly reduced in islets exposed to hyperglycaemia in the primary recipient. These findings are interpreted as showing that long-term exposure of islets to hyperglycaemia results in B-cell loss. In the second experiment 400 islets were transplanted into either a long-term (6 months) diabetic or a normal rat. Two weeks later the composite kidney/islet graft was transplanted into a normal rat. The insulin content was measured after a further 6 months and no significant difference was found, whether the primary recipient was diabetic or not. These results are interpreted as showing that islet graft implantation is not impaired in long-term diabetic rats.