Clinical studies suggest that children respond to head injury with more pronounced cerebral edema and hyperemia than do adults. We hypothesized that these age-related differences could be demonstrated in an animal model. Anesthetized and ventilated mature (2–3 months) and immature (3.5–4.5 weeks) male Wistar rats were traumatized by weight drop onto the exposed right parietal cortex. Trauma severity was adjusted to keep the ratio of force to brain weight constant. This resulted in an energy delivered to the brain of about 9 × 10³ ergs · mm⁻² · g⁻¹ brain in both age groups. Percent right hemispheric brain water (%RBW) was measured at 2, 24, 48, and 168 h posttrauma. Infarct area, intracranial pressure (ICP), and ¹⁴C-iodoantipyrine autoradiographic local cerebral blood flow (ICBF) were measured at 2 h or 24 h posttrauma. In mature rats, %RBW was unchanged at 2 h, but increased at 24 and 48 h (both p < 0.05). In immature rats, %RBW increased at 2 h and remained elevated at 24 and 48 h (all p < 0.05). Traumatic infarct area as a percent of hemispheric area at 24 h did not differ between age groups. In mature rats, at 2 h posttrauma ICBF was reduced (p < 0.05) in 16 of 17 regions but in only 4 of 17 regions in immature rats. ICBF as a percent of age-matched control values showed a greater reduction in mature vs immature rats in 9 of 16 regions (p < 0.05). ICP increased at 24 h posttrauma in both age groups. In immature rats posttrauma, brain water increased earlier and cerebral hypoperfusion was less marked than in mature rats.