Cyclic GMP is a key second messenger, acting in many signal transduction pathways in animal systems. Studies on the role of cGMP in plant cells last only a decade and indicate that it is a significant element in the plant signal transduction. It has been shown that cGMP is involved in the pathway underlying the defensive reaction of HR phytochrome signal transduction pathway and reactions necessary in the regulation of plant stomata. Like in animals, plants cGMP signal inducers seem to be NO and natriuretic peptides. Although many known animal guanylate cyclases, the system has not been identified in plants generating cGMP.