The brain is dependent on glucose as a primary energy substrate, but is capable of utilizing ketones such as b-hydroxybutyrate and acetoacetate, as occurs with fasting, starvation, or chronic feeding of a ketogenic diet. The relationship between changes in cerebral metabolic rates of glucose (CMR glc ) and degree or duration of ketosis remains uncertain. To investigate if CMR glc decreases with chronic ketosis, 2-[18 F]fluoro-2-deoxy-D-glucose in combination with positron emission tomography, was applied in anesthetized young adult rats fed 3 weeks of either standard or ketogenic diets. Cerebral metabolic rates of glucose (mmol/min per 100 g) was determined in the cerebral cortex and cerebellum using Gjedde-Patlak analysis. The average CMR glc significantly decreased in the cerebral cortex (23.0 ± 4.9 versus 32.9 ± 4.7) and cerebellum (29.3 ± 8.6 versus 41.2 ± 6.4) with increased plasma ketone bodies in the ketotic rats compared with standard diet group. The reduction of CMR glc in both brain regions correlates linearly by B9% for each 1 mmol/L increase of total plasma ketone bodies (0.3 to 6.3 mmol/L). Together with our meta-analysis, these data revealed that the degree and duration of ketosis has a major role in determining the corresponding change in CMR glc with ketosis.