Cerebral metabolic rate for glucose (CMRglc) was studied in rats using [6-14C]glucose. After intravenous injection, the radioactivity of the parietal cortex was corrected for loss of labeled CO2 and divided by the integral of the arterial plasma glucose concentration, determined during tracer circulation. Treatment with insulin, resulting in plasma glucose concentrations less than 2.6 mmol/l, reduced CMRglc to 64% of the values found in control animals. CMRglc did not change in animals with acute hyperglycemia produced by intraperitoneal injection of a glucose solution or in diabetes-prone rats with or without insulin treatment.