BACKGROUND: The serotonergic system during pregnancy plays an important role not only in carbohydrate metabolism, but also in the laying and regulation of the fetoplacental complex, growth and development of the fetus. The study of the expression of placental serotonin 5-HT2A receptor and serotonin transporter (SERT) in gestational diabetes mellitus is foremost for scrutinizing the pathogenesis of perinatal complications, as it may allow for finding new opportunities for their prevention and correction. AIM: The aim of this study was to compare the expression patterns of the serotonin 5-HT2A receptor and SERT in placental tissue in gestational diabetes mellitus and in normal pregnancy. MATERIALS AND METHODS: This comparative cohort study included pregnant women with gestational diabetes mellitus (n = 6) and patients with normal pregnancy (n = 10). The expression of serotonin 5-HT2A receptor (Abcam, USA) and SERT (Bioss Antibodies, USA) was studied in placenta samples from the both study groups by immunohistochemical method. Morphometric analysis was performed using the VideoTest-Morphology 5.2 program (Videotest Ltd., Russia). RESULTS: The relative area of SERT expression in the placenta in gestational diabetes mellitus was higher compared to normal pregnancy (p < 0.001). The relative areas of expression of the serotonin 5-HT2A receptor in the placenta did not differ between the study groups (p = 0.5). CONCLUSIONS: Higher SERT expression in the placentas of patients with gestational diabetes mellitus compared to those from women with normal pregnancies may reflect the level of tension of compensatory mechanisms in gestational diabetes mellitus and the effect of insulin therapy on these mechanisms.