Endothelial cells, neutrophils, macrophages, and probably various other cells synthesize nitric oxide or a labile NO-containing compound from L-arginine. NO and various NO-containing compounds activate the cytosolic guanylate cyclase, which catalyzes the formation of the second messenger cyclic GMP. This cyclic nucleotide is involved in the regulations of certain target cells as vascular smooth muscle and platelets. Thereby, NO or a labile NO-containing compound and the cytosolic guanylate cyclase are components of an intercellular signalling system.