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Background: Cyclosporine (CsA), being used as a immunosuppressant is known to have deleterious effects on the liver and kidney, but the harmful effect on glucose tolerance has not been clearly elucidated. This study was undertaken to determine whether the CsA affected peripheral insulin sensitivity in streptozotocin (STZ)-induced diabetic Sprague-Dawley rats. Methods: After the daily treatment of CsA (10mg/kg, i.p.) for 2 weeks, glucose tolerance tests were carried out by the intraperitoneal administration of glucose alone or in conjunction with insulin (5 U/kg, s.c.). The glucose tolerance and peripheral insulin sensitivity were determined by measuring the deremental area under the time-lasma glucose concentration curve (AUC; mg-min/mL) according to the trapezoidal rule. The plasma glucose levels (mg/dL) were measured by a glucose analyzer at 0, 10, 30, 60, 90 and 120min after glucose load (2 g/kg). The STZ-diabetic rats were divided into thre groups (GLU- as control, INS+GLU- and CsA+INS+GLU-treated group, n 7 in each groups). Results: In STZ-diabetic rats, the AUC 0-120 of the CsA+INS+GLU-treated group was significantly (p<0.01) lower than those of the control group (48.6% of control), but significantly (p<0.03) higher thain those of the INS+GLUtreated group (28.1% of control). Conclusions: These results suggest that intraperitoneal injection of CsA gives rise to a deterioration of glucose metabolism which is probably due to a decrease of insulin sensitivity of peripheral tissue in STZ-diabetic rats.