Alterations in the cerebral energy supply are likely to cause cerebral function disturbances. Fasting is a suitable method for studying the energy metabolism. As the cerebrospinal fluid (CSF) compartment reflects the brain metabolism, data in CSF might give information about the metabolism of fuel substrates in brain. We compared the biochemical data on several fuel-related components in blood and CSF at the end of a 40-hours fast of epileptic children with unknown origin of epilepsy (aged 6-15 years) with the values of a reference group of children. In children with primary generalized epilepsy no abnormalities were found. In children with complex partial epilepsy many significant abnormalities were found, such as low blood lactate and alanine and low CSF ketones and CSF blood ratio for ketones. The possible significance of the observed abnormalities are discussed.