The authors studied the effects of a standardized mild-moderate hypoglycemic stimulus (glucose clamp) on brain functional magnetic resonance imaging (fMRI) responses to median nerve stimulation in anesthetized rats. In the baseline period (plasma glucose 6.6 +/- 0.3 mmol/L), the MR signal changes induced by median nerve activation were determined within a fixed region of the somatosensory cortex from preinfusion activation maps. Subsequently, insulin and a variable glucose infusion were administered to decrease plasma glucose. The goal was to produce a stable hypoglycemic plateau (2.8 +/- 0.2 mmol/L) for 30 minutes. Thereafter, plasma glucose was restored to euglycemic levels (6.0 +/- 0.3 mmol/L). In the early phase of insulin infusion (15 to 30 minutes), before hypoglycemia was reached (4.7 +/- 0.3 mmol/L), the activation signal was unchanged. However, once the hypoglycemic plateau was achieved, the activation signal was significantly decreased to 57 +/- 6% of the preinfusion value. Control regions in the brain that were not activated showed no significant changes in MR signal intensity. Upon return to euglycemia, the activation signal change increased to within 10% of the original level. No significant activation changes were noted during euglycemic hyperinsulinemic clamp experiments. The authors concluded that fMRI can detect alterations in cerebral function because of insulin-induced hypoglycemia. The signal changes observed in fMRI activation experiments were sensitive to blood glucose levels and might reflect increases in brain metabolism that are limited by substrate deprivation during hypoglycemia.