The influence of 5-hydroxytryptamine (5-HT) on the functional development and chemical sensitivity of the hippocampus neurons has been studied in newborn rats under the conditions of long-term (up to 35 days) in vitro cultivation. The regular introduction of 5-HT in the nutrient medium resulted in an earlier formation of spontaneous pulse activity of neurons and an increase in the number of active cells in the explants. The neurons with periodic (burst or grouped) activity characteristic of the hippocampus neurons in situ predominated in these cultures. 5-HT was shown to inhibit the activity of the most (88%) neurons grown in the medium with 5-HT and this effect was blocked by LSD. The data obtained allow to consider 5-HT as an endogenous factor regulating the postnatal development of the rat hippocampus.