Serotonin (5-HT) stimulates smooth muscle cell growth through 5-HT receptors and the 5-HT transporter (5-HTT), and has been associated with pulmonary hypertension (PH). Platelet-derived growth factor receptors (PDGFR) have also been associated with PH. We present evidence for the first time that 5-HT transactivates PDGFR through the 5-HTT in pulmonary artery (PA) SMCs. Inhibition of PDGFR kinase with imatinib or AG1296 blocks 5-HT-stimulated PDGFR phosphorylation. 5-HTT inhibitors and the Na⁺/K⁺-ATPase inhibitor ouabain, but not 5-HT2 and 5-HT1B/1D receptor inhibitors, block PDGFR activation by 5-HT. Notably, 5-HTT binds the PDGFR upon 5-HT stimulation and the 5-HTT inhibitor fluoxetine blocks both the binding and PDGDR activation. Activation of PDGFR may occur through oxidation of a catalytic cysteine of tyrosine phosphatase. 5-HT-activated PDGFR phosphorylation is blocked by the antioxidant N-acetyl-L-cysteine and the NADPH oxidase inhibitor, DPI. Inhibition of PDGFR kinase with imatinib or AG1296 significantly inhibits SMC proliferation and migration induced by 5-HT in vitro. Infusion of 5-HT by miniosmotic pumps enhances PDGFR activation in mouse lung in vivo. In summary, these results demonstrate that 5-HT transactivates PDGFR in PASMCs leading to SMC proliferation and migration, and may be an important signaling pathway in the production of PH in vivo.--Liu, Y., Li, M., Warburton, R. R., Hill, N. S., Fanburg, B. L. The 5-HT transporter transactivates the PDGF receptor in pulmonary artery smooth muscle cells.