Changes in intracranial pressure (ICP) resulting from head-down tilt (HDT) were studied in rabbits, and a possible role of edema formation in the change of ICP was examined. Animals were anesthetized with pentobarbital sodium and artificially ventilated. ICP was continuously monitored through a catheter inserted into the subarachnoid space. It increased depending on the tilt angle and decreased when the tilt angle was reduced. ICP elevated from 4.6 +/- 0.7 mmHg (mean +/- standard error of the mean) at horizontal prone position to 13.7 +/- 1.0 mmHg immediately after the onset of 45 degrees HDT and gradually reduced toward the pre-HDT baseline in the next 8 h. ICP decreased below the pre-HDT baseline value immediately after returning to the horizontal prone position, and gradually increased toward the baseline during the 2 h of recovery period. Histological examination (HE stain) demonstrated that exposure to 8 h of HDT did not cause remarkable edema in either the gray matter or the white matter in rabbits. Water content and specific gravity of brain tissues both were increased in the HDT group in comparison with the control group. These results suggest that edema formation plays little role in the elevation of ICP during the acute phase of HDT in rabbits.