Summary Single 17‐day foetal mouse pancreases, cultured for 15 days in RPMI 1640 or DME containing either 1 or 4 g/1 glucose, were grafted under the kidney capsule of syngeneic mice made diabetic with Streptozotocin. Islets cultured in media containing 1 g/l glucose secreted less insulin in vitro than those grown in media containing 4 g/1 glucose. After transplantation, the blood glucose levels of diabetic mice grafted with islets grown in media containing 1 g/l glucose decreased to normal limits, whereas mice grafted with islets grown in media with 4 g/l glucose never became normoglycaemic. Histologically, grafts of tissue from high glucose cultures were smaller and had less well organized islet tissue than grafts from tissue maintained in physiological glucose concentrations. Islets cultured in DME secreted more insulin in vitro than those cultured in RPMI 1640, but the type of culture medium had no significant effect on subsequent graft function. These data support the concept that chronic exposure of foetal islets to high glucose concentrations may be deleterious to their subsequent function as grafts despite the fact that they secrete more insulin in vitro.