The present study was undertaken to characterize the formation of ischemic brain edema using diffusion-weighted and T2-weighted magnetic resonance imaging in a rat model of focal ischemia. The extent of edema formation was measured from multislice diffusion-weighted and T2-weighted spin-echo images acquired at various times after ischemia. The spin-spin relaxation time (T2) and the apparent diffusion coefficient in normal and ischemic tissue were also determined. The results show that on the diffusion-weighted images the lesion was clearly visible at 30 minutes after ischemia, while on the T2-weighted images it became increasingly evident after 2-3 hours. On both types of images the hyperintense area increased in size over the first 48 hours. After 1 week the hyperintensity on the diffusion-weighted images rapidly disappeared and evolved as a hypointense lesion in the chronic phase. These results confirm the high sensitivity of diffusion-weighted MRI for the detection of early ischemia. The temporal course of the edema observed on T2W-images is in agreement with the reported increase of total water content occurring in this model. The increase of the lesion observed on the diffusion-weighted images during the first 2 days points to an aggravation of cytotoxic edema that parallels the changes in free water shown by the T2-weighted images. It is shown that the highly elevated T2's of the infarcted area several days after ischemia can substantially contaminate the diffusion-weighted images.