Endothelial cells are known to contain both soluble and particulate guanylate cyclase, but the functional role of cyclic guanosine monophosphate (cGMP) in endothelial cells remains unknown. We have investigated the effects of 8-bromo-cGMP on endothelium-dependent relaxations to acetylcholine, substance P, ATP, and the calcium ionophore A23187, and on endothelium-independent relaxations to sodium nitroprusside and glyceryl trinitrate (GTN). The ability of each of these agents to relax phenylephrine-preconstricted rings of rabbit aorta was tested in the absence and presence of 8-bromo-cGMP. In the presence of 8-bromo-cGMP, a greater concentration of phenylephrine had to be used to produce a similar level of tone and then endothelium-dependent relaxations to acetylcholine and substance P were inhibited, whereas endothelium-dependent relaxations to ATP and A23187 were unaffected. Endothelium-independent relaxations to sodium nitroprusside and GTN were only inhibited at the highest concentrations of nitroprusside and GTN. These results suggest that: (a) increasing GMP levels in endothelial cells inhibit agonist-induced release of endothelium-derived relaxing factor (EDRF); (b) a negative feedback mechanism may exist whereby EDRF stimulates soluble guanylate cyclase in endothelial cells to inhibit its own release; and (c) ATP does not induce EDRF release via phosphoinositol hydrolysis.