The objective of this study was to detect the threshold between reversible and irreversible secondary insult of hypoxia and hypotension following closed head injury as measured by MRI. Adult Sprague rats were separated into 3 groups: I: Sham (n = 6), II: Trauma and hypoxia coupled with mild hypotension of 40-50 mmHg (n = 6), III: Trauma and hypoxia coupled with severe hypotension of 30-40 mmHg (n = 6). The measurement of brain water content (BWC) was based on T1, whereas the differentiation between reversible and irreversible secondary insult on the measurement apparent diffusion coefficient (ADC). The ADCs in both trauma and secondary insult groups decreased rapidly from a control level of 0.68 +/- 0.5 x 10(-3) to significantly different minimum levels of 0.52 +/- 0.5 x 10(-3) in Group II and 0.42 +/- 0.5 x 10(-3) mm2/second in Group III at 30 minutes. In Group II rats there was a complete recovery in ADC as well as in their clinical conditions, whereas ADC in Group III rats remained at the minimum level and the animals were brain dead. The BWC was also significantly different at four hours post injury (Group II: 80.3 +/- 0.7%, Group III: 81.8 +/- 0.8%). The data lead the authors to suggest that the threshold between reversible and irreversible posttraumatic secondary insult is very narrow, and the measurement of ADC can provide information that will enable the clinician to identify critical threshold beyond which recovery is not possible.