In spite of intense recent investigation of the physiological and pathophysiological roles of endogenously produced nitric oxide (.NO) in mammalian systems, little quantitative information exists concerning the diffusion of this small nonelectrolyte from its source (NO synthase) to its targets of action. I present here a conceptual framework for analyzing the intracellular and intercellular diffusion and reaction of free .NO, using kinetic modeling and calculations of the diffusibility of .NO and its reactions in aqueous solution based on published data. If the half-life of .NO is greater than approximately 25 msec and the rates of reaction of .NO with its targets are slower than its diffusion or reaction with O2 (for which there is experimental evidence in at least some systems), then (i) .NO acts in vivo in a mostly paracrine fashion for a collection of .NO-producing cells, (ii) .NO diffuses to significant concentrations at distances relatively far removed from a single .NO-producing cell, and (iii) localized sites of vascularization will scavenge .NO (and thus decrease its actions) at distances many cell diameters away from that site. These conclusions have important implications with regard to the mechanism of endothelium-dependent relaxation, the autocrine vs. paracrine actions of .NO, and the role of the spatial relationship between specific sites of .NO formation and neighboring blood vessels in .NO-effected and -affected neuronal signal transmission.