The main initiator of the development of PAH is the development of endothelial dysfunction of the pulmonary vessels. The cause of this dysfunction is associated with functional disorders of the serotonin system. This research is devoted to study the role of the serotonin system in the development mechanism of PAH in children with CHD. We examined 30 young children (from 1 month to 2 years) with congenital heart defects (CHD), which were divided into 3 groups: group I - 12 children with CHD complicated by pulmonary arterial hypertension (PAH); Group II - 9 children with CHD, not complicated by PAH; Group III - 9 relatively healthy children (control). The serotonin content of platelets, serum and serotonin transporter (SERT) in platelets was determined. To determine serotonin and its carrier, a quantitative enzyme-linked immunosorbent assay (Elisa kit) was used. The study was performed using the Serotonin ELISA diagnostic kit, IBL Hamburg at the automatic ELISA of the Evolis robotic station (BioRad). The results of our studies showed a pronounced (p <0.001) increase in SERT concentration in platelets in the group of children with CHD complicated by PAH compared with the group of children with CHD and the control group. The amount of serotonin in serum tended to increase in this group compared with the control groups and CHD, and the amount of serotonin in platelets tended to decrease. It is likely that an increase in the serotonin transporter in platelets against the background of an increase in serotonin in the blood plasma and a decrease in serotonin in the platelets indicates the inadequacy of SERT activity as a serotonin carrier. However, by activating the proliferation of smooth muscle cells in the pulmonary artery, SERT leads to the development of PAH in children with congenital heart defects. It is necessary to continue research in this direction, which will reveal the molecular mechanisms by which SERT regulates PASMC proliferation and clarify the role of serotonin and its transporter in the development of PAH in children with CHD.