The present experiments were undertaken to study how preischemic hyperglycemia, which is known to exaggerate ischemic damage and to trigger delayed postischemic seizures, affects the bioenergetic state and the intracellular pH (pH_i) of brain tissue at early (6 h) and late (18 h) recirculation times. To that end, normo- and hyperglycemic rats were subjected to 10 min of forebrain ischemia, and neocortical tissue was frozen in situ for analyses of labile energy metabolites. Animals with preischemic hyperglycemia failed to show a postischemic reduction of the phosphorylation state of the adenine nucleotide pool, or a rise in tissue lactate content, nor did they show a change in tissue redox state. However, the hyperglycemia led to a rise in phosphocreatine (PCr) content after 6 h of recirculation. Calculations of intracellular pH (pH_i) from the creatine kinase (CK) equilibrium showed a rise in pH_i above normal, a finding which was supported by a limited number of 5,5-dimethyl[2-¹⁴C]oxazolidine-2,4-dione (DMO) measurements. The preischemic hyperglycemia also blunted the postischemic rise in tissue glycogen content, which is usually observed in normoglycemic rats. The results thus fail to reveal that the hyperglycemia-triggered, masssive exaggeration of ischemic brain damage, which is heralded by generalized seizures after 18–24 h of recirculation, is preceded by mitochondrial dysfunction of a degree which affects the bioenergetic state or the redox potential of the tissue. However, the results suggest that the hyperglycemia enhances and/or prolongs the postischemic alkalosis. It is discussed whether the rise in pH contributes to the mitochondrial dysfunction which subsequently develops.