To delineate the importance of cyclic GMP (cGMP) as a second messenger for signal transduction within the cell, catalytic and regulatory features of the proteins involved in the synthesis and degradation of cGMP, i.e., guanylate cyclases and phosphodiesterases, are described. Characteristic features of cGMP-dependent protein kinases as intracellular receptors for cGMP and the contribution of this enzyme to the physiological functions of cGMP in the cardiovascular system are discussed.