Micropuncture studies were performed in three groups of euvolemic male Munich-Wistar rats: 8 control rats, 7 severely hyperglycemic rats made diabetic with streptozotocin (60 mg/kg, i.v.), and 6 moderately hyperglycemic rats made diabetic by the same method but given 2 U of NPH insulin daily. Glucose concentrations at the time of micropuncture study averaged 115 +/- (sem) 10, 565 +/- 12, and 375 +/- 23 mg/dl, respectively. Single nephron GFR (SNGFR) values were significantly lower (28.8 +/- 1.9nl/min) in severely hyperglycemic rats than they were in controls (48.9 +/- 3.8). This reduction in SNGFR was due mainly to a fall in glomerular plasma flow rate ((Q(A)). In contrast, moderately hyperglycemic rats exhibited glomerular hyperfiltration, with SNGFR values averaging 69.0 +/- 8.0 nl/min. This hyperfiltration, with resulted from elevations in values for Q(A) and glomerular transcapillary hydraulic pressure difference (delta P) to levels significantly above control. These alterations in SNGFR in severely hyperglycemic and moderately hyperglycemic rats, relative to controls, were paralleled by changes in whole kidney GFR and mimic the changes in GFR observed in diabetic patients with analogous degrees of hyperglycemia. Measurements of blood volumes in separate groups of control, severely, and moderately hyperglycemic rats revealed equivalent absolute blood volumes in all three conditions and increased blood volumes, relative to body weight, in both groups of hyperglycemic rats. Thus, SNGFR is increased in diabetic rats with moderate hyperglycemia but decreased in those with severe hyperglycemia, and these changes are not simply related to variations in circulating blood volume.