The value of the nuclear magnetic resonance image (MRI) was investigated in the acute stage of experimental cerebral ischemia. The MRI system employed was designed for clinical use, and the superconducting magnet was operated at a field strength of 1.5 tesla. Ischemic insult was made by transorbital occlusion of the middle cerebral artery (MCA) permanently in 4 cats and temporarily in 2 cats. After MCA occlusion the regional cerebral blood flow (rCBF) was measured on the affected cortex, and 5 cats with rCBF below 10 ml/100 g/min and one with rCBF over 15 ml/100 g/ min were studied. In the permanent occlusion group, MRI was performed every 2 hours from 4 to 12 hours after MCA occlusion and another MRI was carried out 20 min after gadoliniumdiethylenetriamine-pent aacetic acid (Gd-DTPA) intravenous administration. The earliest changes were found 6 to 8 hours after MCA occlusion on the spin echo image (repetition time=1.4 sec, echo time=70 msec) in 3 cats with severe ischemia. It was postulated that the ischemic lesion could be depicted less than 6 hours on more T2-weighted images. The increased intensity area was markedly enhanced with Gd-DTPA 12 hours after occlusion. In the recirculation group, the increased intensity area was observed on enhanced MRI in a cat with recirculation as early as one hour after MCA occlusion, although it was not found on the plain MRI. In the other cat with recirculation after 2 hours' occlusion, definite lesion was found in all parameter images without enhancement. The results suggest that changes in cerebral ischemia can be obtained on the MRI earlier than X-ray computed tomography, and that it may be possible to determine the severity of the ischemic brain injury by the MRI findings.