Guanylyl cyclase activity exists in the soluble and particulate fractions of most tissue homogenates. Recently, the mRNA encoding members of the enzyme family from both the soluble and particulate fraction have been cloned. The soluble form exists as a heterodimer, both subunits containing cyclase catalytic-like amino acid sequences. Both subunits must be expressed to observe enzyme activity. The particulate (plasma membrane) forms contain a single putative transmembrane domain that separates an extracellular, ligand-binding domain from an intracellular, protein kinase-like and cyclase catalytic region. The protein kinase-like domain appears to serve as an important regulatory element of the receptor.Key words: receptors, plasma membrane, guanylyl cyclase, cGMP, protein kinase.