We investigated the long-term (up to 1 week) relationships between the duration of cerebral ischemia and postischemic energy metabolic profile, pH, and tissue edema in the rat. Ten rats each were subjected to 8 or 12 min of forebrain ischemia induced by bicarotid occlusion concurrent with systemic hypotension, and the results were compared with those of 10 sham-operated rat controls. In vivo 31P nuclear magnetic resonance spectroscopy was performed prior to ischemia and at intervals up to 168 h after ischemia. Cerebral edema (measured by specific gravity) was assessed prior to ischemia and at 24, 72, and 168 h after ischemia. The data revealed significant differences in the brain tissue pH profile over time between the ischemic groups (p less than 0.03). The 12-min ischemic animals exhibited brain tissue alkalosis (pH = 7.27 +/- 0.12) at 24 h compared with both sham (pH = 7.09 +/- 0.08) at 24 h and preischemic (pH = 7.06 +/- 0.04) pH values. The pH remained alkalotic (pH = 7.23 +/- 0.15) through the 48-h time period. In contrast, in the 8-min group, the onset of alkalosis was delayed until 48 h after ischemia (pH = 7.24 +/- 0.15), and pH remained alkalotic for only 24 h. No difference in high-energy phosphate metabolism was detected between groups. A different time dependence of tissue pH and specific gravity changes after 12 min of ischemia was detected. The present study suggests that the duration of an ischemic event marks the time of onset of brain tissue alkalosis and its duration and that cerebral edema alone cannot explain the pH changes.