Urinary nitrite/nitrate concentrations provides reasonable estimate of whole-body nitric oxide (NO) synthesis. We tested the hypothesis that NO synthesis undergo maturational changes after birth in 10 preterm infants (mean GA 33.2 ± 2.5 w. range 30-37; BW 2160 ± 445 g, range 1440-3010) admitted to NICU. The newborns were delivered spontaneously (n=5) and by cesarian section (n=5). No patient had neonatal asphyxia (Apgar score at 1′; 8±0.8; at 5′; 8.5 ± 1), respiratory disease, diarrhea, or conditions reported to alter urinary nitrite/nitrate concentrations. Daily 8-hour urine samples were collected during the first, third and fifth day of life and kept at -20 °C until the assay was performed. Nitrite/nitrate concentrations were measured spectrophotometrically according to a modification of a method described previously (Bartholomew et al., Food Chem Toxicol 22:511, 1984). Nitrate reductase prepared from E. Coli ATCC 25922 (Difco) was used to convert nitrate to nitrite. Total nitrite was quantified colorimetrically after reduction with the Grieas reagent (Green et al, Annal Biochem 126:131, 1982). Creatinine was also measured by the Ektachem method based on the enzymaric reaction of creatinine with creatinine iminohydrolase. RESULTS. Nitrite/nitrate concentrations(mean±SD in μmol/mmol creatinine) were: 1st day 123.36±75.17; 3rd day: 135.01±63.22; 5th day 109.94±40.36. No correlation was found between nitrite/nitrate concentrations and gestationa age, postnatal age, birthweight or type of delivery. Urine nitrite/nitrate concentration varied widely as previously reported (Dollberg et al. Pediatr Res 37:31, 1995). Mean values remained substantially stable during the first 5 days of postnatal age. Urinary nitrite/nitrate levels are considered a reasonable estimate of in vivo conversion of L-arginine to NO. The present preliminary results suggest that NO synthesis from L-arginine may not undergo substantial developmental changes during the first 5 days postnatally.