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Object. The goal of this study was to examine the mechanical properties of living rat intracranial contents and corresponding brain structural alterations following parietal cerebral cortex contusion.Methods. After being anesthetized, young adult rats were subjected to parietal craniotomy followed by cortical contusion using a calibrated weight-drop method. Magnetic resonance imaging was used to visualize the contusion. At the site of contusion, instrumented force-controlled indentation was performed 2 hours to 21 days later on the intact dural surface. The force-deformation (stress-strain) relationship was used to calculate elastic (indentation modulus) and strain changes over time, and constant hold or cyclic stress was used to evaluate viscoelastic deformation. These measurements were followed by histological studies.Results. At contusion sites, the indentation modulus was significantly decreased at 1-3 days and tended to be above control values at 21 days. Multicycle indentation showed that the brain tended to accumulate more strain (an indicator of viscosity) by 1 day after the contusion. Imaging and histological studies showed local edema and hemorrhage at 6 hours to 3 days and accumulation of reactive astrocytes, which began at 3 days and was pronounced by 21 days.Conclusions. The viscoelastic properties of living rat brain change following contusion. Initially, edema and tissue necrosis occur, and the brain becomes less elastic and less viscous. Later, along with undergoing reactive astroglial changes, the brain tends to become stiffer than normal. These quantitative data, which are related to the physical changes in the brain following trauma and which reflect subjective impressions upon palpation, will be useful for understanding emerging diagnostic tools such as magnetic resonance elastography.