In this study, the role of cyclic GMP, the end product of the NO-cyclic GMP signalling pathway, in the regulation of ecNOS was investigated. Bovine pulmonary endothelial cells were exposed to 8-bromo-cyclic GMP and its effect on NO production, ecNOS protein, and mRNA levels was analyzed. Endothelial cells on exposure to 8-bromo-cyclic GMP produced significantly increased amounts of NO, detected as increased cyclic GMP in cocultures with vascular smooth muscle cells both under basal conditions and with agonist stimulation. 8-Bromo-cyclic GMP significantly increased the ecNOS protein and mRNA levels as detected on Western and Northern blots respectively. This 8-bromo-cyclic GMP mediated increase of NO production, ecNOS protein and mRNA levels suggests that cyclic GMP up-regulates the expression of ecNOS. Thus, there may be an intercellular feedback mechanism involved at the molecular level in the expression of the NO-cyclic GMP signalling pathway in blood vessels.