The aim of the present study was to measure capillary blood pressure and interstitial pressure in transplanted pancreatic islets and to correlate these measurements to capillary and tubular pressures in the adjacent kidney. For this purpose, 250 syngeneic islets were implanted under the renal capsule of WF rats and studied 1, 2, or 6 months after transplantation. Some of the animals studied after 1 and 2 months were streptozotocin (STZ)-induced diabetic. Measurements were performed during basal conditions or after an acute glucose-stimulation of insulin release. The hydrostatic pressures were determined in vivo by direct micropuncture. The islet transplant capillary pressure in normoglycemic animals was 6.9 +/- 0.4 mmHg (n = 9), 10.0 +/- 0.8 mmHg (n = 7), and 12.4 +/- 0.8 mmHg (n = 7) when measured 1, 2, and 6 months after implantation, respectively. Previous data from our laboratory showed that the normal capillary pressure of native rat pancreatic islets is approximately 3 mmHg. The blood pressure in kidney peritubular capillaries was 10-12 mmHg in both transplanted and control animals. Islet transplant interstitial pressures were 4-6 mmHg in the normoglycemic recipients at 1, 2, and 6 months after transplantation. Acute glucose stimulation had no effect on islet transplant interstitial pressure or peritubular or transplant capillary blood pressures. Capillary pressures in the islet grafts were slightly increased 1 month after transplantation in STZ-induced diabetic rats, and this was associated with an increased blood perfusion of the transplants. However, 2 months after transplantation there were no differences in transplant capillary blood pressure between diabetic and normoglycemic animals. The graft interstitial pressure was, on the contrary, decreased in the diabetic animals 2 months after transplantation. We concluded that the capillary blood pressure in islets implanted under the renal capsule was similar to that of the implantation organ, which was three to four times higher than that normally found in native islets.