Intracranial pressure (ICP) and other related phenomena were monitored in 42 cases with various pathologic conditions to study the pathogenesis of rapid variations in ICP. Pressure waves appeared tc be closely related to changing respiratory patterns; the initial rises in pressure usually corresponded to suppressed movement in respiration, while the pressure reduction corresponded to hyperpnea and/or tachypnea. A transient fall in blood pressure also appeared coincident with the initiation of plateau-like waves. Such synchronized changes in vital signs associated with occurrence of pressure waves suggested o possible role of brain stem dysfunction. In experimental pressure waves induced in 24 dogs with intracerebral expanding lesions, initiation of each pressure oscillation was also accompanied by a small fall in systemic arterial pressure as well a, weakened respiration. No distinct alterations in endotidal CO2 and blood gas were detected. During these experiments, levels of sinus pressure revealed a positively synchronized oscillation corresponding tc small changes in ICP. At somewhat higher pressure levels, however, typical long-lasting pressure waves were accompanied by continued falls in sinus pressure. Presumably brain stem dysfunction affects the initiation of pressure waves during intracranial hypertension and, further, the mode of venous outflow from the cranial cavity influences the duration and pattern of rapid ICP variations.