It has been previously reported that the antioxidant compound nordihydroguaiaretic acid (NDGA) increases endothelial nitric oxide synthase (eNOS) expression in cultured bovine aortic endothelial cells. However, the exact mechanism for this effect was unresolved. Thus, the purpose of this study was to further elucidate the effect of NDGA on eNOS protein expression and enzymatic activity in fetal pulmonary arterial endothelial cells (FPAECs), and to identify the transcription factors involved in this regulation. Following overnight exposure to 0-32 microM NDGA, we observed a 2- to 2.5-fold increase in eNOS protein expression in FPAECs, with a similar increase observed in eNOS activity. For eNOS gene promoter analysis, we initially used two promoter-reporter constructs: a 1.6 kb promoter fragment and an 840 bp construct, both of which include an AP-1-specific binding site. NDGA exposure induced a significant increase in eNOS promoter activity in both constructs. However, the NDGA-mediated increase was abolished when we used either a truncated promoter construct lacking the AP-1 element or a construct in which the AP-1 binding site was mutated. AP-1 binding efficiency was also determined by electrophoretic mobility shift assay, where we observed an increase in AP-1 binding in FPAECs treated with NDGA while the binding of AP-1 was found to be decreased when a mutated AP-1 consensus sequence was used. Further, supershift analyses indicated that the AP-1 complex consisted of c-Jun and FosB. We therefore conclude that NDGA antioxidant activity regulates eNOS expression via AP-1 and that antioxidant therapy could potentially be used to increase eNOS expression in diseases, such as persistent pulmonary hypertension of the newborn, where eNOS expression and activity are known to be reduced.