In vivo 1H-NMR imaging was employed to study dynamic changes in the status of tissue water as a function of time after mechanical brain injury induced by partial unilateral transection of the fimbria fornix (FF) in the rat brain and was correlated with histology. Changes in the brain tissue were reproducibly found in distinct regions which were exclusively located in the lesioned hemisphere. The most pronounced changes concerned the lateral ventricle. Ventricular enlargement became evident posterior to the site of transection after a few hours and was maximal after 2-4 days. At later time points the posterior ventricular expansion was reduced. The lateral ventricle anterior to the site of transection was significantly enlarged from day 1 and continued to expand for up to 7 months. Tissue response at the site of transection, mainly involving the hippocampal formation and the thalamus, was first manifested after 24 h, while signs of progressive tissue degeneration were apparent in the long term.