Background/Aims: Cyclic 3′,5′-guanosine monophosphate (cGMP), a central molecule in the phototransduction cascade, is also involved in a number of other physiological processes in the retina, like stimulating the absorption of subretinal fluid by activating the retinal pigment epithelium (RPE) cell pump. The aim of this study was to quantify cGMP synthesis by RPE cells and to investigate the role of two separate enzymatic pathways (soluble versus particulate guanylyl cyclase) in its production. Methods: cGMP expression was evaluated by immunochemistry and radioimmunoassay following culture of the D407 RPE cell line in the presence of a nonselective phosphodiesterase inhibitor (IBMX), in combination with the particulate guanylyl cyclase stimulator atrial natriuretic peptide (ANP) or the soluble guanylyl cyclase stimulator sodium nitroprusside (SNP). Results: Stimulation of the particulate guanylyl cyclase in RPE cells with ANP resulted in high intra- and extracellular cGMP levels. Stimulation of the soluble guanylyl cyclase by SNP resulted in a slight elevation of cGMP levels compared to controls. Conclusions: These results show that cultured human RPE cells are capable of producing cGMP and that most cGMP is generated following stimulation of the particulate guanylyl cyclase pathway.