We designed a simple technique for surgical excision of the pineal body in mice. Such can be accomplished in 10 min by a skilled worker and there are no lesions whatever. In pinealectomized El-strain mice the appearance of convulsions was inhibited up to 30 days, while intact El-mice were all fallen into convulsions by being shaken up and down on a flat carton. Pentazole-induced convulsions propagate from one dd-mouse to another in an aggregated state, but such propagation did not occur in pinealectomized animals. Exploratory movements of pinealectomized dd-mice increased as compared to intact animals and the increase was always more extensively observed in El-mice in which, even in the intact animals, the exploratory movements were much more than those of dd-mice. From the above-mentioned experiment, it is considered that the pineal body plays a role not only in initiating but also in propagating convulsions, and that it may have a depressive action on motor activities in mice.