Background and Purpose—Many patients with acute stroke are excluded from receiving thrombolysis agents within thenecessary time limit (3 or 6 hours from stroke onset) because they or their family members are unable provide the time of stroke onset. Brain tissue sodium concentration ([Na﹢]) increases gradually and incessantly during the initial hours of experimental focal cerebral ischemia but only in severely damaged brain regions. We propose that this steady increase in [Na﹢] can be used to estimate the time after arterial occlusion in the rat middle cerebral artery occlusion model of ischemic stroke. Methods—Sixteen anesthetized Sprague-Dawley rats underwent permanent middle cerebral artery occlusion combined with bilateral common artery occlusion. After 100 to 450 minutes, diffusion-weighted MRI was used to generate apparent diffusion coefficient (ADC) maps, cerebral blood flow (CBF) was determined with 14C-iodoantipyrine (in asubset of 7 animals), and the brain was frozen. Autoradiographic CBF sections and punch samples for Na1 analysis were obtained from the brain at the same level of the MR image. Severely at risk regions were identified with an ADC of﹤520 μm2/s and, in the subset, with both ADC of﹤520 μm2/s and CBF of﹤40 mL• 100 g﹣¹ • min﹣¹.Results—Both CBF and the ADC dropped quickly and remained stable in the initial hours after ischemic onset. Linear regression revealed strong linearity between [Na﹢] and time after onset, with a slope of 0.95 or 1.00 (mEq/kg DW)/min, with both ADC and ADC-plus-CBF criteria, respectively. The 95% CIs at 180 and 360 minutes were between 41 and 52 minutes.Conclusions—The time after ischemic onset can be estimated with this 2-step process. First, ADC and CBF are used toidentify severely endangered regions. Second, the [Na﹢] in these regions is used to estimate time after onset. The favorable 95% CIs at the time limits for thrombolytic therapy and the availability of measurements of ADC, CBF, and [Na1] in humans through the use of MRI suggest that this time-estimation scheme could be used to assess the appropriateness of thrombolysis for patients who do not know when the stroke occurred.