To clarify the mechanism of glucose intolerance induced by FK506, a novel immunosuppressant, 5 or 10 mg/kg/day of FK506 was dosed orally to rats for 2 weeks, and ^&lt125&gtI-insulin binding to the erythrocytes, plasma glucose and insulin levels, and pancreatic insulin content were examined. Insulin binding to the erythrocytes of rat dosed with FK506 was similar to that to erythrocytes of the placebo control; Scatchard analysis confirmed that FK506 did not cause damage to the insulin receptor of the erythrocytes. Contrarily, FK506 caused a clear decrease of pancreatic insulin content as well as a slight decrease of plasma insulin level. The results suggest that the glucose intolerance induced by FK506 is associated with a decrease of insulin secretion, but is not associated with impairment of the insulin receptor.