A study was designed to clarify the influence of the hippocampus upon tolerance by electric shock. Forty-eight male rats were used, of which 14 rats had their hippocampal tissue on both sides removed through an opening in the parieto-occipital cortex (hippocampal group), 17 rats received damage to the parieto-occipital cortex only (cortical control group), and 17 rats served as normal control animals. After 24 hours fasting with water ad libitum, each animal was restrained on a plate with added electric shock (4 mA A.C., 1.5 sec in a duration, and once per minute in average) to the tail for the last 24 hours without food and water. The mortality in each animal group and the mean survival time of the dead animal during the repitition of electric shock were calculated. Results obtained were as follows: 1. The mortality was lower significantly in the hippocampal group than in the two control groups. 2. The mean survival time of the dead animal was longer insignificantly in the hippocampal group than in the two control groups. The inference from the above results is that the hippocampus exerts a inhibitory influence upon tolerance by electric shock.