SummaryThe electrical activity of the hippccampus has been studied in unrestrained rabbits with electrodes chronically implanted in the brain. Simultaneous observations on the electrographical and behavioral chenges in free‐moving animals revealed that definite behavioral xanifestatiom were correlated with four levels of electrical activity of the hippocampus: (1) The excited state. The animal is exploring and its hippocampal activity shows regular slow waves at a frequency of 8 cycles per second. (2) The relaxed state. The animal is eating or licking and its hippocampal activity consists of 7 to 5 cycles per second slow waves. (3) The drowsy state. The animal lies down and its hippocampal slow waves are interspersed by fast activity, (4) The asleep state. The animal lies in a lateral position and closes its eyes and its hippocampal activity displays fast activity intermixed with irregular slow waves around 2 to 3 cycles per second. On the other hand, the electrical activity of the neocortex differs from that of the hippocampus in showing no definite correlation with the bzhavioral patterns. The dissociation between hippocampal and neocortical electrical activity has been pointed out, and its mechanism has been discussed from a phylogenetical point of vie