Changes in apparent diffusion coefficients (ADC) were compared with alterations of adenosine triphosphate (ATP) concentration and pH in different phases of transient focal cerebral ischemia to study the ADC threshold for breakdown of energy metabolism and tissue acidosis during ischemia and reperfusion. Male Wistar rats underwent 1 hour of middle cerebral artery occlusion without recirculation (n = 3) or with 1 hour (n = 4) or 10 hours of reperfusion (n=5) inside the magnet, using a remotely controlled thread occlusion model. ADC maps were calculated from diffusion-weighted images and normalized to the preischemic value to obtain relative ADC maps. Hemispheric lesion volume (HLV) was determined on the last relative ADC maps at different relative ADC thresholds and was compared to the HLV measured by ATP depletion and by tissue acidosis. The HLVs, defined by ATP depletion and tissue acidosis, were 26.0% +/- 10.6% and 38.1% +/- 6.5% at the end of ischemia, 3.3% +/- 2.4% and 4.8% +/- 3.5% after 1 hour of reperfusion, and 11.2% +/- 4.7% and 10.9% +/- 5.2% after 10 hours of recirculation, respectively. The relative ADC thresholds for energy failure were consistently approximately 77% of the control value in the three different groups. The threshold for tissue acidosis was higher at the end of ischemia (86% of control) but was similar to the results obtained for ATP depletion after 1 hour (78% of control) and 10 hours (76% of control) of recirculation. These results indicate that the described relative ADC threshold of approximately 77% of control provides a good estimate for the breakdown of energy metabolism not only during middle cerebral artery occlusion but also at the early phase of reperfusion, when recovery of energy metabolism is expected to occur, or some hours later, when development of secondary energy failure was described.