NADPH oxidase is a major source of the superoxide produced in cardiovascular tissues. The expression of NOX1, a catalytic subunit of NADPH oxidase, is induced by various vasoactive factors, including angiotensin II, prostaglandin (PG) F(2alpha), and platelet-derived growth factor (PDGF). It was reported previously that the inducible expression of NOX1 is governed by the activating transcription factor-1 (ATF-1)-myocyte enhancer factor 2B (MEF2B) cascade downstream of phosphoinositide 3 (PI3) kinase. It was also reported that extracellular signal-regulated kinase (ERK) 1/2 is involved in the expression of NOX1. To further clarify the factors involved in NOX1 induction downstream of ERK1/2, the promoter region of the NOX1 gene was analyzed. A consensus activator protein-1 (AP-1) site was found at -98/-92 in the 5'-flanking region of the rat NOX1 gene. The introduction of mutations at this site abolished PGF(2alpha)-induced transcriptional activation in a luciferase assay. Electrophoresis mobility shift assays demonstrated that PGF(2alpha) and PDGF augmented the binding of JunB to this sequence. PD98059, an inhibitor of MAPK/ERK kinase, suppressed the expression of JunB induced by PGF(2alpha) or PDGF. These results suggest that the ERK1/2-JunB pathway is a key regulator of the inducible expression of the NOX1 gene in vascular smooth muscle cells.