Nitric oxide (NO), has been shown to affect the function of leukocytes and the overall host immune response to infection, sepsis and septic shock. NO is synthesized by three different isoforms of NO synthase (NOS), two are constitutively expressed (NOS1 and NOS3) and another (NOS2) is inducible with specific stimulus. While both NOS2 and NOS3 have been shown to have a role in sepsis and septic shock, less is known about the role of NOS1 in the host response to sepsis. There is some evidence to suggest that NOS1 might have a very important role in the host response to infection. The enzyme is constitutively expressed not only in neuronal cells in the brain and spinal cord, but also in the microvasculature and epithelium of the gastrointestinal tract and kidney, bronchial epithelium, myocytes of skeletal muscle, mast cells in skin, and neutrophils. Under baseline conditions and during sterile peritonitis, mice congenitally lacking NOS1 (NOS1-deficient) have increased leukocyte rolling and adhesion to the endothelium of postcapillary venules and increased leukocyte migration into the peritoneal cavity. We are investigating the effects of NOS1 on extravascular neutrophil recruitment, bacterial clearance, and inflammatory tissue injury during polymicrobial peritonitis, sepsis, and septic shock. At baseline, NOS1-deficient animals have higher neutrophil and lower platelet counts compared with wild type animals. We found that, although NOS1-deficient animals have reported increased migration of neutrophils into the peritoneum, in the setting of live bacterial peritonitis and sepsis, genetic deficiency of NOS1 is detrimental and increased mortality. Such detrimental effect in survival was also seen in a model of lipopolysaccharide (LPS) challenge where in addition we were able to determine that there are alterations in other cell populations beside neutrophils that traffic in and out of the peritoneum during an LPS challenge. The mechanisms of this increased mortality in NOS1-deficient animals are yet unknown but it appears that NOS1-deficient have higher systemic bacterial counts in blood compared to wild type animals suggesting that animals deficient in NOS1 may have decreased bacterial clearance. We are now studying the mechanism whereby NOS1 impacts on survival and bacterial clearance during sepsis and septic shock. We have learned that NOS1 may alter cytokine and chemokine expression profiles during polymicrobial infections which might in turn alter the hosts ability to clear bacteria during infection.