Events associated with an evolving cerebral infarction were studied using multiple magnetic resonance imaging (MRI) techniques at 4.7 T in a rat model of middle cerebral artery occlusion. High resolution perfusion images revealed a core of absent perfusion surrounded by a zone of slow, but measurable perfusion. Only the core of severest perfusion deficit demonstrated restricted water diffusion as early as 1 hr, consistent with "cytotoxic" cellular edema in the most vulnerable region. Within 24 hours, the area of restricted diffusion encompassed the entire region destined to become infarcted. In spin-echo images, hypointensity, likely reflecting deoxygenated hemoglobin, was visible in the ischemic hemisphere. Edema accumulated over 72 hr primarily in the surrounding slowly perfused rim, consistent with the concept of "vasogenic" edema. These studies demonstrate that multimodal MRI can visualize events which define the ischemic penumbra--deoxygenation, maintenance of transmembrane ionic gradients, reduced flow, and delayed cell death. These experiments noninvasively visualized differential hemodynamic and biochemical processes within the core and perifocal penumbra and will allow quantitation over time of the relationship between blood flow, cytotoxicity and edema in stroke.