Evidence indicates that there are multiple sites in the hippocampal pyramidal cell dendrites which are capable of producing spikes. The multiple spikes generated there summate and form an excessive, prolonged depolarization known as the inactivation response (IR). Despite the fact that the IR consists of multiple components, the IR behaves in all-or-nothing fashion. This is explained as follows. Namely, in the dendrites there is a portion which is of low threshold for spike generation (the dendritic trigger zone) whereas the other portions in the dendrites are of high threshold. The spike of the dendritic trigger zone triggers spikes in the high threshold portions of the dendrites, resulting in the IR. Since the spike in the trigger zone is produced in all-or-nothing fashion, the IR thereby triggered is of necessity triggered in all-or-nothing fashion.<BR>It is known that the spike originating in the somatic trigger zone conducts along the axon therewith carrying the informations arising within the cell onto the target cell. This is called the first carrier system for the sake of convenience. On the other hand, the IR is thought to produce a spike burst down in the axon while inactivating the soma spikes. A corollary from this is that once the IR is produced, the burst discharge thereby set up in the axon becomes the only means through which the informations are carried to the next cell. Consequently, the IR-triggered information carrying system is called the second carrier system, in contradistinction to the first carrier system mentioned above. Evidently then, the neuron with well developed dendrites has two independent systems for carrying informations to its target cell. It should be noted that although the second carrier system originates in the dendrites, not only the axo-dendritic input, but also the axo-somatic input can put this system into operation as evidenced by generation of the IR by current injection into the cell body. On the other hand, both inputs are also capable of activating the first carrier system, provided that the IR is not produced. In the second carrier system the informations are translated into the changes of the discharge pattern of the IR-triggered burst while in the first carrier system they are expressed as those of the discharge pattern of the single spike of the somatic trigger zone. A possibility has been pointed out that the parameters of the IR-triggered burst, i. e., the carrier of the second carrier system, are altered through a process leading to a plastic change.<BR>The changes in the electrophysiological properties of the dendrites during seizure, including kindled seizure, have been discussed, especially from the viewpoint of plasticity.