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Increased nitric oxide (NO) levels are thought to play an important role in the pathophysiology of the systemic inflammatory response syndrome (SIRS) which is caused by disseminated vascular endothelial damage. Clinical studies have shown that urinary nitrite (NO2-) and nitrate (NO3-) excretions can be utilized as indexes of NO formation. The SIRS and NO relationship was investigated in 15 neonates with SIRS, gestational age 32.5 +/- 4.4 weeks and weight 1,737 +/- 753 g. The control group comprised 18 neonates with a gestational age of 32.8 +/- 3.5 weeks and a weight of 1,778 +/- 538 g. There was no significant difference in birth weights and gestational ages between the two groups (p > 0.05 and p > 0.05). The urinary nitrite levels obtained in the subjects were normalized for urinary creatinine concentrations. The mean urinary nitrite levels in the control group neonates were found to be 4.22 +/- 1.8 micromol/mmol creatinine on the 1st day, 4.09 +/- 2.28 on the 2nd, 3.62 +/- 1.6 on the 3rd, and 4.01 +/- 1.12 micromol/mmol creatinine on the 7th day. There were no statistically significant differences between these levels (p > 0.05). We determined urinary levels of nitrite in neonates in the study group within the first 24 h of SIRS symptoms and found these levels (18.35 +/- 11.16 micromol nitrite/mmol creatinine) to be elevated as compared with those of the control subjects on the 7th day of life (p < 0.0005). In conclusion, urinary nitrite excretion is significantly elevated in neonates with SIRS due to septic events, and these results suggest that NO might play a part in SIRS.