The effect of induced hypocapnia was observed, during intraventricular pressure monitoring, in five patients with chronically increased intracranial pressure (ICP) and in five baboons with acutely raised ICP. The volume-pressure response (VPR) was used as a measure of intracranial elastance (inverse compliance), an index of residual compensatory capacity. The VPR is the acute increase in mean ICP that occurs in response to a constant volume increment to ventricular cerebrospinal fluid. In the patients, hypocapnia produced a decrease in the VPR that was proportional to the decrease in ICP; in the baboons, ICP was substantially decreased with minimal change in the VPR. Both observations indicate that hypocapnia, though it decreases ICP, does not selectively decrease intracranial elastance. This may be a limiting factor in the use of hypocapnia in the management of raised ICP.