This work was designed to investigate the effect of an additional hyperglycemia on the subsequent in vivo insulin secretion in rats with various degrees of glucose intolerance. Four groups of rats received a unique injection of a low concentration of streptozotocin (STZ): 20, 27, 30, or 35 mg/kg corresponding, respectively, to STZ 20, STZ 27, STZ 30, or STZ 35 rats. Control rats were injected with citrate buffer. In all STZ groups, impaired glucose tolerance and insulin secretion were observed. These defects were roughly proportional to STZ concentration. Three weeks after STZ administration, hyperglycemia (17 mM) was produced by a 48-h glucose infusion via an indwelling catheter. Insulin secretion in response to glucose was investigated 3 h after stopping glucose infusion, by performing iv glucose tolerance tests. Insulin secretion in response to glucose doubled in control rats previously submitted to glucose infusion, and still more in rats with mild glucose intolerance (three and four times higher in STZ 20 and STZ 27 rats, respectively). By contrast, glucose infusion increased insulin secretion only slightly in STZ 30 rats and it was unchanged in STZ 35 rats. These data show that prolonged hyperglycemia has an improving effect on insulin secretion in rats with mild glucose intolerance, whereas the potentiating effect of previous hyperglycemia is lost in rats with more severe glucose intolerance.