To study the speed of development of cerebral edema in an animal model, five 2-year-old male anesthetized Merino sheep were impacted in the left temporal region by a humane stunner. Following the induction of blunt craniocerebral trauma a highly significant increase in intracranial pressure (ICP) values occurred relative to control animals (ANOVA, p<0.001). An immediate increase in ICP to values over 20 mmHg occurred within the first 30 min, and by 60 min, mean ICP was over 25 mmHg (p<0.05 versus controls). ICP continued to increase with time such that by 4h after injury, values were consistently greater than 30 mmHg (p<0.001 versus controls). The mean brain tissue oxygenation (P(bt)O(2)) in control animals over the 4h monitoring period was 52+/-8 mmHg with a highly significant (ANOVA, p<0.001) and early decrease in P(bt)O(2) recorded in impacted animals following craniocerebral trauma. By 30 min after injury, P(bt)O(2) was approximately 35 mmHg, while by 60 min, it had decreased to 20+/-2 mmHg (p<0.001 versus controls). The P(bt)O(2) continued to decline with time such that by 4h, the value was 14+/-2 mmHg (p<0.01 versus controls), or 27% of the mean control values. These data demonstrate that vasoreactive changes with increased blood volume and interstitial transfer of fluid may occur rapidly within brain tissue after blunt trauma. Detection of cerebral swelling at autopsy may not, therefore, imply prolonged survival.