Guanylate cyclase from particulate and soluble fractions of tissue homogenates will be purified on a preparative scale. The particulate form of the enzyme has been purified on an analytical scale from sea urchin sperm, while the soluble form has been purified on an analytical scale from rat lung. The soluble form will be purified on a preparative scale from a large animal tissue such as lung, liver or heart. Purified forms of guanylate cyclase will be studied with respect to physical characteristics, amino acid content, kinetic properties, and mechanisms of regulation.