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Moderate unilateral cerebral ischemia was produced by microembolism in 24 adult cats. Two million plastic microspheres with a diameter of 15 +/- 5 microns were injected into the left common carotid artery via the lingual artery. The physiological and metabolic responses to embolism were accessed by electrocorticography and by determining the cerebral energy state. Embolism caused an immediate slowing and voltage reduction of the ipsilateral electrocorticogram with a gradual recovery after 30 to 60 min. Some animals also had an immediate and short depression of the contralateral electrocorticogram. In spite of the market functional suppression, metabolites of the cerebral energy-producing metabolism in most of the animals changed only slightly. In the embolized hemisphere pyruvate increased from 0.06 to 0.10 mumol/g and lactate from 1.9 to 4.6 mumol/g within 5 min after embolization and remained at this level during the 4 h observation period. Phosphocreatine, adenosine triphosphate and the energy charge of the adenylate pool remained uncharged during this period. However, there was a slight increase of ATP in the non-embolized hemisphere during the early postembolic period. In two animals, the initial slowing of the electrocorticogram recurred and spread to the contralateral hemisphere, followed by bilateral flattening after a few hours. This delayed functional deterioration was accomplished by complete loss of energy-rich phosphates. These animals also had a progressive increase of cerebrospinal fluid (CSF) pressure and considerable brain swelling with cerebellar herniation after 4 h. It is concluded that unilateral cerebral embolism in the above concentration leads only to a slight increase of anerobic glycolysis without significant perturbation of the cerebral energy state, unless progressive brain swelling with cerebrellat herniation supervenes. This supports previous findings, that brain edema and not initial ischemia is the main pathogenetic factor for tissue damage in cerebral microembolism.