Purpose: To investigate MRI for noninvasive autopsy by means of measurements of serial changes in relaxation parameters of the rat brain during the postmortem interval. Materials and Methods: Postmortem relaxometry measurements were performed before and hourly after death for 24 h on five control rats and five rats that underwent middle cerebral artery occlusion. Analyses were performed on representative regions of gray, white, and mixed gray/ white matter structures. Results: Significant decreases in both T-1 and T-2 values were measured in all areas in the control group within 24 h of death. In the stroke animals, T-2 differences between normal and ischemic striatal tissue decreased by 11 +/- 4% (P < 0.01), with a complete convergence of T-2 values observed between ischemic striatal tissue and nonischemic cortical tissue. Conclusion: Lesion conspicuity and the ability to differentiate between different tissue compartments are significantly affected by postmortem interval, and alterations to pulse timing parameters will be necessary if the sensitivity of MRI to detect central nervous system diseases in postmortem tissue is to be maintained. Indeed in the case of stroke at least, convergence of T-2 values with normal tissue post mortem indicates that T-1-weighted images may be more sensitive to the presence of such lesions.