AbstractHippocampal and neocortical EEG activity was recorded from 48 unanesthetized, intra‐uterine, guinea‐pig fetuses of 35 to 65 days gestational age. The earliest hippocampal electrical activity appeared at around 35 days (i.e., at approximately the same time as such activity appears in the lower brain stem, and earlier than it appears in the neocortex). This indicates that functional maturation of the brain begins almost simultaneously in large parts of the so‐called old brain, not only in its bulbar parts, and spreads to more superficial structures with age. The first activity had a random frequency distribution, whereas more regular periodicity developed later. From 44 days hippocampal bursts and spindles were recorded, especially during hypoxia or fetal distress. The bursts seemed to bear a relation to the reanimation process after hy