The role of nitric oxide in cellular signaling in the past 22 years has become one of the most rapidly growing areas in biology with more than 20,000 publications to date. Nitric oxide is a gas and free radical with an unshared electron that can regulate an ever-growing list of biological processes. In many instances nitric oxide mediates its biological effects by activating guanylyl cyclase and increasing cyclic GMP synthesis from GTP. However, the list of effects of nitric oxide that are independent of cyclic GMP is also growing at a rapid rate. For example, nitric oxide can interact with transition metals such as iron, thiol groups, other free radicals, oxygen, superoxide anion, unsaturated fatty acids and other molecules. Some of these reactions result in the oxidation of nitric oxide to nitrite and nitrate to terminate its effect, while other reactions can lead to altered protein structure, function, and/or catalytic capacity. These diverse effects of nitric oxide that are either cyclic GMP dependent or independent can alter and regulate important physiological and biochemical events in cell regulation and function. Nitric oxide can function as an intracellular messenger, an autacoid, a paracrine substance, a neurotransmitter, or as a hormone that can be carried to distant sites for effects. Thus, it is a unique simple molecule with an array of signaling functions. However, as with any messenger molecule, there can be too little or too much of the substance and pathological events result. Some of the methods to regulate either nitric oxide formation, metabolism, or function have been in clinical use for more than a century as with the use of organic nitrates and nitroglycerin in angina pectoris that was initiated in the 1870's. Current and future research with nitric oxide and cyclic GMP will undoubtedly expand the clinicians' therapeutic armamentarium to manage a number of important diseases by perturbing nitric oxide and cyclic GMP formation and metabolism. Such promise and expectations have obviously fueled the interests in these signaling molecules for a growing list of potential therapeutic applications.