Plasma nitrite and nitrate and urine nitrite concentrations. (a) Plasma nitrite concentrations were significantly higher in adults, cord blood, and term infants than in preterm infants on the first day after birth (*=difference from preterm infants on day 1, P < 0.05). Nitrite concentrations increased over the first 20 d after birth, with days 10, 15, and 20 greater than day 1 (**P < 0.01). (b) On day 1, plasma nitrate concentrations in term infants were significantly lower than those of adults, cord blood, and preterm infants (* = difference from term infants, P < 0.05). The variation in preterm nitrate levels over the first 20 d after birth was also significant (P < 0.01), although post-hoc analysis did not detect a significant difference between day 1 and any subsequent time point. (c) Urine nitrite concentrations in preterm and term infants on day 1 were significantly higher than those of adults (**P < 0.001). Urinary nitrite concentrations of preterm infants increased over the first 20 d of life, with concentrations on days 15 and 20 being significantly greater than days 1 and 5 (*P < 0.05).