Aldosterone has been shown to stimulate renal TGF-beta(1) expression. However, the mechanisms for aldosterone-induced TGF-beta(1) expression have not been clearly determined in mesangial cells. We examined the role of extracellular-signal regulated kinase 1 and 2 (ERK1/2), c-Jun N-terminal kinase (JNK) and activator protein-1 (AP-1) in the aldosterone-induced TGF-beta(1) expression in rat mesangial cells. TGF-beta(1) protein in the conditioned medium released from rat mesangial cells was measured by sandwich ELISA, TGF-beta(1) mRNA expression was analyzed by Northern blotting, AP-1 DNA binding activity was measured by EMSA and the ERK1/2, JNK activity was analyzed by western blotting. Aldosterone significantly stimulated TGF-beta(1) protein production and TGF-beta(1) mRNA expression in mesangial cells in a dose-dependent manner. Aldosterone significantly increased AP-1 DNA binding activity in mesangial cells. Pre-treatment of cells with AP-1 inhibitor, curcumin, blocked aldosterone-induced AP-1 DNA binding activity as well as aldosterone-induced TGF-beta(1) production. Aldosterone increased phosphorylation of ERK1/2 and JNK in mesangial cells. Pre-treatment of cells with ERK1/2 inhibitor, PD98059, or JNK inhibitor, SP600125 significantly inhibited aldosterone-induced ERK1/2 and JNK activity and subsequently TGF-beta(1) production, respectively. We conclude that aldosterone-induced TGF-beta(1) expression in mesangial cells is regulated by the ERK1/2, JNK and AP-1 intracellular signaling pathways.