This special volume provides up-to-date information on research dealing with the regulation of the biosynthesis and degradation of cyclic GMP and with the regulation of physiological systems by cyclic GMP. The publication of this volume could not have come at a more appropriate time, with significant work in this area having led to our current understanding of the role of nitric oxide in signal transduction, and the 1998 Nobel Prize in Medicine and Physiology for Furchgott, Murad and Ignarro. Moreover, the introduction in early 1998 of sildenafil (Viagra), a selective inhibitor of a cyclic GMP phosphodiesterase, for the treatment of erectile dysfunction in men represents the first successful therapeutic application of an agent designed to alter the activity of a molecular target in a cyclic GMP pathway.