The mediators responsible for maintenance of the hyperdynamic state and the low systemic vascular resistance (SVR) observed in sepsis have not been elucidated. Nitric oxide (. N= O) is a mediator with numerous functions, including regulation of vascular tone and a role in macrophage-mediated cytostasis and microbiostasis. Thirty-nine critically ill trauma and septic patients were studied to determine the relationship between. N= O production and the hyperdynamic state. high plasma levels of NO2-/NO3-(the stable end products of. N= O) were observed in septic patients (p less than 0.02). Low SVR and high endotoxin levels were associated with high NO2-/NO3-values (p= 0.029, p= 0.002). Changes in. N= O levels may mediate the vasodilation seen in sepsis. Low NO2-/NO3-levels were observed in trauma patients (p less than 0.001) and remained low even in the presence of sepsis (p= 0.001).