A model of closed head injury in rats was developed using a calibrated weight-drop device. The development of edema was studied in various brain regions (cerebral hemispheres, brain stem, cerebellum) using a linear specific gravity gradient column. Regional brain tissue density was measured within 1 min, at 15 and 60 min, 18 h, 4 and 10 days after injury to the left cerebral hemisphere, and was compared with values in sham-operated and control rats. Significant edema (i.e., reduced specific gravity) occurred only in the traumatized hemisphere and was maximal at 18 h. A neurologic severity score (NSS) was developed to evaluate the status of the rat after injury. Specific gravity was significantly correlated with NSS at 18 h after injury. The affected hemisphere displayed hemorrhagic lesions as early as one hour post head trauma (HT), which evolved into hemorrhagic necrosis at 18 h. A pathologic score, evaluated 18 h post HT based on size and severity of the lesion, was correlated with the NSS and evaluated for each rat at one hour and 18 h postimpact. This correlation was found to be highly significant. This model of brain injury may be useful in future studies on the effects of therapeutic agents.