We have previously observed that human islets, transplanted under the kidney capsule of hyperglycemic nude mice, show a longlasting impairment in glucose-induced insulin release. To investigate the cause(s) of this phenomenon, we transplanted human islets into normoglycemic or alloxan-diabetic nude mice for a 4- to 6-week period. In a third experimental group, aimed at evaluating reversibility of hyperglycemia effects, diabetic nude mice bearing a human islet graft were cured by a second intrasplenic transplant of mouse islets, and the human islets were exposed to a further 2 weeks of normoglycemia. Four to 6 weeks of hyperglycemia induced a severe impairment of glucose- and arginine-induced insulin release, as demonstrated by perfusion of the graft-bearing kidney. This defective release was not restored by a subsequent 2-week period of normoglycemia, and it was accompanied by normal (pro)insulin biosynthesis, glucose oxidation, and expression of insulin messenger RNA. Taken together with our previous study, these observations indicate that impaired glucose metabolism, depletion of insulin messenger RNA, decreased (pro)insulin biosynthesis, increased glycogen accumulation, and depletion of insulin reserves cannot explain the deleterious effects of the diabetic state on human islet insulin release. This, and the similar inhibition of glucose- and arginine-induced insulin release, suggest that prolonged hyperglycemia may exert its deleterious effect on insulin release at a step distal to closure of ATP-sensitive K-channels.