Although NO appears important in rodent immune responses, its involvement in the human immune system is unclear. We report that human NK cells express constitutive endothelial NO synthase mRNA and protein, but not detectable levels of inducible NO synthase. They produce NO following activation by coculture with target cells or cross-linking with anti-CD16 mAb, and production is increased in the presence of IL-2. N-monomethyl-l-arginine (l-NMA), a NOS inhibitor, partially inhibited NK cell lysis of four different target cells (< 40% inhibition at 500 μM l-NMA), but not granule release following coculture with target cells, or Fas ligand induction following cross-linking with anti-CD16 mAb. However, l-NMA augmented apoptosis of NK cells induced by activation through CD16 ligation or coculture with K562. An NO donor, S-nitroso-N-acetylpenicillamine (SNAP), suppressed apoptosis of NK cells induced by CD16 cross-linking or coculture with target cells, suggesting that endogenous NO production is involved in protection of NK cells from activation-induced apoptosis, thereby maintaining NK activity. SNAP also suppressed, and l-NMA enhanced, expression of TNF-α, reported to be involved in activation-induced NK cell death, in response to CD16 cross-linking. Suppression of anti-CD16-induced apoptosis by SNAP was reversed by the addition of rTNF-α. DNA-binding activity of the transcription factor, NF-AT, which is involved in TNF-α induction upon ligation of CD16, was inhibited by SNAP and enhanced by l-NMA. Our results suggest that down-regulation of TNF-α expression, possibly due to suppression of NF-AT activation, is a mechanism by which endogenous NO protects NK cells from activation-induced apoptosis, and maintains lytic capacity.