We used MR imaging to locate and monitor in vivo the pathological events taking place 2 to 4 weeks after unilateral striatal injections of ibotenic acid (IA) in the Papio papio baboon. As early as 2 weeks after IA injections, excitotoxic lesions in the caudate and the putamen were directly visualized on T1-weighted images as small areas of low signal intensity. On T2-weighted images, the lesion sites were visualized as areas of high-intensity signal, spreading over larger areas than the corresponding regions in T1-weighted images. These alterations of T2-values in the lesioned striatum persisted 4 weeks after surgery. However, as the striatal degeneration progressed from 2 to 4 weeks after lesion, the size of the areas of high signal intensity on T2-weighted images decreased, whereas the same regions appeared essentially unmodified on T1-weighted images. A marked enlargement of the ipsilateral lateral ventricle (a characteristic of excitotoxic striatal lesions) could be detected 4 weeks after surgery, on both axial T1- and T2-weighted images. Comparisons of MR images with postmortem anatomical data indicated that areas of increased T1 values corresponded to regions of severe neuronal depletion (a direct result of the excitotoxic lesion), whereas areas of increased T2 values corresponded to regions of increased content in astrocytes and ferritin and probably in the early period following lesion (2 weeks) to a superimposed edema.