This volume is dedicated to the topic of cyclic GMP. Chapters include discussions on the guanylyl cyclase and phosphodiesterase isoenzyme families for cyclic GMP synthesis and hydrolysis, cyclic GMP-dependent protein kinases, and various hormones and ligands that regulate cyclic GMP formation and/or metabolism. Several chapters also deal with some of the effects of cyclic GMP on other second messengers such as calcium ion transport and smooth muscle relaxation. Some clinical studies with cyclic GMP and atrial natriuretic peptide are also discussed. The last chapter raises many important questions in the field that remain to be addressed. – Isoforms of guanylyl cyclase and phosphodiesterase isoenzyme families for cyclic GMP synthesis and hydrolysis- Cyclic GMP-dependent protein kinase- Hormones and ligands that regulate GMP formation and/or metabolism- Effects of cyclic GMP on other second messengers and some functions such as smooth muscle relaxation and ion transport- Clinical studies with cyclic GMP and atrial natriuretic peptide- Important questions and experiments for the future