Cyclosporine A (CsA) and FK506 increase endothelial nitric oxide synthase (eNOS) mRNA expression in cultured bovine aortic endothelial cells (BAEC). CsA appears to increase eNOS mRNA levels mainly by increasing the rate of transcription, although a small contribution of mRNA stabilization could not be ruled out. CsA and FK506 induced an increase of ROS synthesis with the fluorescent probe used, DHR123. The ROS generating system glucose oxidase (GO) increased the expression of eNOS mRNA in BAEC. This upregulation of eNOS mRNA by CsA or GO was abrogated by catalase. As AP-1 is a redox-sensitive transcription factor and the bovine eNOS promoter has an AP-1 consensus sequence, a role of this factor in the up-regulation of eNOS mRNA was studied. Electrophoretic mobility shift assays were consistent with an increase in AP-1 DNA-binding activity in BAEC treated with CsA or glucose oxidase. The potential participation of ROS and the transcription factor AP-1 in the regulation of eNOS gene expression is suggested.