Simultaneous recordings of the epidural pressure (EDP) and the ventricular fluid pressure (VFP) were studied in 1 dog and 3 patients, including 2 with severe intracranial hypertension. The simultaneous recordings were continued for periods of 2.5 h to 3 days. Results of such comparative studies have to our knowledge previously not been reported. Rapid variations in the VFP tracing were always reflected by similar variations in the EDP tracing. Under standard conditions there was a linear correlation between the two pressures, the EDP being constantly higher than the VFP. The difference between the two pressures increased with increasing pressures. Thus, at VFP levels between 10 and 20 mm Hg this difference was about 15 mm Hg and at VFP levels between 50 and 60 mm Hg about 25 mm Hg. The slope of the regression line differed from patient to patient but was fairly constant in a given patient. Deviations from the typical slopes were observed under certain conditions, e.g. after lowering a high intracranial pressure by removal of cerebrospinal fluid.