This paper deals with results of the effect of air pressure applied to the dural sac for varying time periods. A device was used consisting of a cylinder containing a piston motivated by a spring. When the spring is released the piston moves through the cylinder. First, air pressure of a given magnitude comes in contact with the dural sac and, as the piston moves farther, the air is permitted to escape at the same time that the air pressure is stopped. By using this technique, changing the mass of the piston and the size of the spring, varying speeds were obtained. A small diaphragm type strain gauge pressure pick-up was used to determine the pressure and its time duration, with a photographic record obtained from a dual beam oscilloscope. In another group of experiments an iron bar was dropped into a column of water in contact with the dural sac through an opening previously made in the skull, into which was screwed a plastic tube. By changing the weight of the cylindric bar and the height from which it was dropped, different time periods and levels of pressure were obtained. These experiments are summarized in 2 tables. It was concluded that the shorter the time duration, the higher the pressure necessary to result in a concussive effect. The longer the time duration, the lower the pressure can be in order to effect a concussion. Concussion resulting from acceleration, deceleration or compression is caused by an increase in intracranial pressure at the time of impact. With compression this increase may be more uniform. With acceleration and deceleration there are pressure gradients in the intracranial cavity.