Recent studies reveal that serotonin (5-hydroxytryptamine, 5-HT) might play a role in the aetiology of gestational diabetes mellitus (GDM). The involvement of the serotonin transporter (SERT) and 5-HT(2A) receptor (5-HT(2A)R) in diabetes has also been suggested. However, placental SERT and 5-HT(2A)R have never been studied in GDM-complicated pregnancies. The aim of this study was to investigate the effect of GDM on the expression of both placental SERT and 5-HT(2A)R. First, immunohistochemical analysis demonstrated the presence of SERT and 5-HT(2A)R proteins in the villous trophoblast and the fetal capillary endothelium of normal term placental tissue. Protein and mRNA expression of SERT and 5-HT(2A)R in the villous cytotrophoblastic and syncytiotrophoblastic cells was further confirmed in primary culture. A significantly (P < 0.05) decreased expression of SERT mRNA (56.3%) and protein (79.7%), and 5-HT(2A)R mRNA (79.1%) and protein (29.1%) was observed in placental tissues from GDM compared with non-GDM pregnancies. These data suggest that SERT and 5-HT(2A)R might be implicated in the aetiology of GDM. Moreover, the presence of SERT and 5-HT(2A)R in villous trophoblastic cells argues in favour of an important role of serotonin in human placental function.