As an important intracellular second messenger,cyclic GMP regulates various functions of cells,such as visual and olfactory signal transduction, vascular smooth muscle relaxation,lymphocyte activation and chemotaxis of reproductive cell.In different kinds of cells,cyclic GMP interacts with cGMp-dependent protein kinases,cGMp-regulated ion channels,cGMp-regulated cyclic nucleotide phosphodiesterases or ADP-ribosyl cyclase respectively.This means that cyclic GMP can regulate cell functions through protein phosphorylation or through pathways not directly related to protein phosphorylation. It appears that the regulatory mechanisms of cyclic GMP are variable and related to the specific subcelluar structure.