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1. The effects of insulin hypoglycaemia on cerebral blood flow and metabolism have been examined in unanaesthetized, unrestrained calves between 1 and 26 days after birth. 2. Cerebral blood flow was measured with an inert gas technique using molecular hydrogen, and cerebral metabolism was quantified by determination of arterio-cerebral venous (A--V) concentration differences for oxygen, glucose, lactate, pyruvate, acetoacetate, beta-D-hydroxybutyrate and ammonia. 3. During normoglycaemia the mean (A--V) difference for glucose was close to one sixth that of oxygen, on a molar basis. A small net loss of pyruvate from the brain was found, but there was no significant (A--V) difference for lactate. Arterial concentrations of acetoacetate and beta-D-hydroxybutyrate were low, and no utilization of ketone bodies by the brain was demonstrated. 4. Moderate hypoglycaemia (arterial plasma glucose concentration 1--2 m-mole/l.) had no measurable effect on either cerebral blood flow or metabolism. 5. During profound hypoglycaemia (arterial plasma glucose concentration less than 1.0 m-mole/l.) cerebral glucose uptake was sufficient to account for only 56% of the cerebral oxygen consumption. Cerebral oxygen consumption fell in comatose animals, but increased during hypoglycaemic convulsions, as did cerebral blood flow. 6. In day-old calves hypoglycaemia was associated with a rise in blood lactate concentration and uptake of lactate by the brain. 7. A net loss of ammonia by the brain was observed during hypoglycaemia in calves at all ages examined. The loss was greater in convulsing than in comatose animals.