It was inevitable that important relationships between two of the most intensely studied topics in biomedical research, apoptosis and nitric oxide (NO), would become apparent. Apoptosis is essential to normal development as well as physiological cell turnover. Although apoptosis in excess can manifest as tissue damage, a failure to undergo apoptosis constitutes pathological cellular overgrowth. It is now evident that NO and its reaction products can either promote or prevent apoptosis in a multitude of settings. The ubiquitous distribution of the NO synthases and the remarkable diffusibility and diverse chemical reactivity of NO in biological systems make this molecule unique among the regulators of apoptosis. Understanding the factors that govern the consequences of NO exposure on cell viability and identifying the conditions in which NO regulation of apoptosis contribute to pathology are topics of considerable interest and potential importance. In this article, we will review the recent observations on NO as a regulator of apoptosis. Apoptosis, or programmed cell death, is distinguished from lytic or necrotic cell death by specific biochemical and structural events (see recent review in Reference 1). Apoptogenic signals trigger cell-specific signaling pathways, including protease activation, followed by the appearance of morphological changes characteristic of cells undergoing apoptosis, including condensation of nuclei and cytoplasm, blebbing of the cytoplasmic membranes, and finally fragmentation into apoptotic bodies that are phagocytosed by neighboring cells. The elucidation of the signaling events in apoptosis is occurring at a rapid pace and includes the identification of the key roles of cysteine proteases (known as caspases), Bcl-2 family members, and mitochondria. Caspases, the mammalian counterpart of ced-3 in Caenorhabditis elegans, are a family of cysteine proteases now known to contain at least 14 homologs. Ectopic expression of any of the caspase family proteases can cause apoptosis; however, not all caspase family proteases have been definitively linked to apoptosis. Caspase family genes encode proenzyme forms that require proteolytic cleavage for activation. Caspases can propagate apoptotic signaling by cleaving/activating other caspases, or they can execute the terminal events in apoptosis by cleaving key death substrates. For example, caspase-9 cleaves/activates caspase-3, whereas caspase-3 cleaves specific target proteins, including poly (ADP-ribose) polymerase (PARP), DNA-dependent kinase, and the inhibitor of the caspase-dependent activated deoxyribonuclease (ICAD). The antiapoptotic effect of compounds that inhibit either the activation or activity of caspase-3–like proteases suggests that apoptosis can be regulated by modification of the protease-signaling cascade. One mechanism by which Bcl-2, itself a substrate for caspase-3, prevents cell death during physiological and pathophysiological processes is through the inhibition of mitochondrial cytochrome C release. The release of cytochrome C results in the activation of caspase-9. Although endogenous inhibitors of caspase activation and activity have been described, none has been shown to be more prevalent than NO. NO is short-lived and is synthesized from L-arginine by the catalytic reaction of NO synthases (NOSs)(reviewed in Reference 2). NOSs are expressed in microorganisms, plants, and mammals, in addition to participating in diverse physiological functions including neurotransmission, regulation of vascular tone, cellular communication, inflammation, and immune responses. The mammalian NOS isoforms include the neuronal type 1 isoform …