The formation of trigeminal motor nucleus (TMN) was studied in the early postnatal period in 21 female Wistar rats which received the serotonin biosynthesis inhibitor para-chloro-phenylalanine at prenatal Day 16 (the period of serotoninergic system formation). It was shown that the serotonin deficit during the prenatal period in rats resulted in the changes of TMN structural organization. In the early postnatal period, the delay of neuropil development, the reduction of cell body size with the partial loss of Nissl substance in some of the neurons, the presence of degenerating neurons with the signs of hyperchromatosis in all the parts of the nucleus, especially in TMN ventromedial part, were detected. At later stages, the destruction of motoneurons became slower, though some of them had morphological abnormalities. With the increase of the postnatal age (by Day 20) the number of motor neurons decreased, apparently, as a result of the gradual intensification of cell death. Simultaneously with the motor neuron degeneration in TMN parts studied, the astrocytic gliosis was observed.