Dorsal hippocampal fragments taken from newborn rats (1--5 days) were grown in rotating tubes and Maximov's chambers both in a common nutritional medium, consisting of placental human serum, Eagle's medium with embryonal extract and addition of 3 mg/ml serotonin into the medium during the course of cultivation. Experimental and control hippocampal explants were examined 3, 5, 7, 9, 12, 14, 16, 18, 20 and 23 days after cultivation by means of light and electron microscopy. Comparative morphological investigation demonstrated that presence of serotonin in small concentrations in the nutritional medium of the hippocampal experimental cultures not only produced an increased growth and accelerated, comparing with control cultures, cytodifferentiation of neuronal and glial cells but also speeded up, to a considerable degree, the process forming glio-neuronal interrelations, myelinization including, and formation of interneuronal synaptic connections in them.