We examined the links between fibrotic and proliferative pathways for the 5-HT_2A receptor in rat mesangial cells. Serotonin (5-hydroxytryptamine, 5-HT) induced transforming growth factor-β1 (TGF-β1) mRNA in a concentration-dependent (peak at 30 nM 5-HT) and time-dependent fashion. For 10 nM 5-HT, the effect was noticeable at 1 h and maximal by 6 h. Inhibition of1) protein kinase C (PKC),2) mitogen- and extracellular signal-regulated kinase kinase (MEK1) with 2′-amino-3′-methoxyflavone (PD-90859), and3) extracellular signal-regulated kinase (ERK) with apigenin attenuated this effect. The effect was blocked by antioxidants,N-acetyl-l-cysteine (NAC) and α-lipoic acid, and mimicked by direct application of H₂O₂. TGF-β1 mRNA induction was also blocked by diphenyleneiodonium and 4-(2-aminoethyl)-benzenesulfonyl fluoride, which inhibit NAD(P)H oxidase, a source of oxidants. 5-HT increased the amount of TGF-β1 protein, validating the mRNA studies and demonstrating that 5-HT potently activates ERK and induces TGF-β1 mRNA and protein in mesangial cells. Mapping studies strongly supported relative positions of the components of the signaling cascade as follow: 5-HT_2A receptor → PKC → NAD(P)H oxidase/reactive oxygen species → MEK → ERK → TGF-β1 mRNA. These studies demonstrate that mitogenic signaling components (PKC, MEK, and oxidants) are directly linked to the regulation of TGF-β1, a key mediator of fibrosis. Thus a single stimulus can direct both proliferative and fibrotic signals in renal mesangial cells.