Tentorial herniation is caused by a pressure gradient between supra and infratentorial compartments and the herniated brain is burdened with a force, which is called shear strain at the edge of the tentorium. The purpose of the present study is to clarify the relationships between increased intracranial pressure, the pressure gradient and the tentorial shear strain. Twenty-three monkeys were used and intracranial pressure was raised by inflation of an epidural balloon placed in the right temporal region. The supratentorial pressure was found to be always higher than the infratentorial pressure and the pressure gradient became greater as intracranial pressure increased. The shear strain at the tentorial edge began to develop as soon as the balloon was expanded, and showed a slow and steady increase even when the intracranial pressure was only slightly increased and the pressure gradient did not have any effect. The degree of the tentorial edge descent observed by X-ray was, however, variable depending upon the animals used. The tentorial edge ceased to descend at the moment the tentorial shear strain was 80-140 mm Hg. At this point and thereafter, the characteristics of both herniated brain and tentorial edge changed from elastic to plastic, and the damage caused by the strained tentorial edge is thought to be tremendously extensive. Our dynamical study of transtentorial herniation shows clearly that the magnitude of shear strain was greater as a local forced pressure than supra and infratentorial pressure.