Acute subdural hematoma (ASDH) gives rise to a mass effect not only by itself but also through unilateral hemispheric swelling. The present study tested the hypothesis that hemispheric swelling is mediated by mechanisms which involve excitatory amino acids (EAAs). After removal of the subdural clot, introduced by homologous blood (0.1-0.2 ml), the % brain water was determined from the formula: ((wet weight--dry weight)/wet weight) x 100. The % brain water of the left hemisphere was significantly greater than that of the right hemisphere during the initial 6 hours after induction of ASDH in animals injected with 0.2 ml blood. A less marked but significant increase was observed in the animals injected with 0.1 ml blood. Systemic pretreatment with kynurenic acid (KYN; 800 mg/kg, i.p.), a broad-spectrum EAA antagonist, attenuated the increase in % brain water in the animals injected with 0.2 ml blood. In order to determine the changes in cerebral metabolism induced by the model of ASDH employed in the present study, we measured the cortical cytochrome oxidase (CYO) activity, a marker of mitochondrial respiration, in a separate group of animals. The CYO activity estimated densitometrically from the histochemical staining was not significantly altered in the animals injected with either 0.1 or 0.2 ml blood, suggesting absence of ischemia. These results indicated that the hemispheric swelling associated with thin ASDHs may be partially mediated by mechanisms other than ischemia, in which EAAs appear to be involved.