Urinary nitrite/nitrate concentrations are reasonable estimates of whole-body nitric oxide (NO) synthesis. L-arginine, the precursor of NO is present in fresh frozen plasma (FFP). Thus, we determined the effect of FFP on NO synthesis in 8 preterm infants (mean GA 33.1 ±2.2 w. range 30-37; BW 2050±350 g, range 1440-2710) admitted to NICU. No patient had perinatal asphyxia, respiratory disease, diarrhea, or conditions reported to alter urinary nitrite/nitrate concentrations. 5 newborns received FFP and 3 had aminoglycoside treatment for suspected but not confirmed infection. Daily 8-hour urine samples were collected during the first 5 days of life and kept at -20°C until the assay was performed. Nitrite/nitrate concentrations were measured spectrophotometrically using 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 reaction with the Griess reagent (Green et al, Annal BIochem 126:131, 1982). Creatinine was measured by the Ektachem method based on the enzymatic reaction of creatinine with creatinine iminohydrolase. RESULTS: Urine nitrite/nitrate concentrations varied widely (range: 38 to 298 μmol/mmol creatinine). No correlation was found between nitrite/nitrate concentrations and gestational age, postnatal age, birthweight or antibiotic treatment. Conversely, a transient but significant decrease in urinary nitrite/nitrate concentrations was found in 5 patients who received fresh frozen plasma (basal values 169.2±71.7, day of transfusion 65±54.1, p<0.005). Our preliminary results suggest that NO synthesis from L-arginine may be depressed by plasma transfusion. In a previous study, this occurred with red blood cells transfusion (Miller et al, Acta Pediatri 82:291, 1993). Further studies are needed to clarify the impact of plasma transfusion on NO metabolism.