This paper reviews new models of nitric-oxide (NO)-related regulation of transepithelial and transendothelial permeability in normal tissues, with emphasis on physiological relevance of the data. Novel data obtained in cultured human epithelial and endothelial cells indicate that NO can originate from different intracellular sources, and can be selectively regulated and employed to activate various intracellular mechanisms that will affect the permeability via different mechanisms. These models may be important for understanding NO regulation of permeability in vivo.