This study assessed and compared the plasma and red blood cell concentrations of carnitine in cord blood samples from preterm (less than or equal to 36 wk, n = 53) and term (greater than or equal to 37 wk, n = 72) neonates. The mean (+/- S.E.) plasma carnitine concentration (PL[C]) was significantly higher in preterm than in term neonates (29.0 +/- 1.8 versus 22.4 +/- 0.8 nmole/ml; P less than 0.001). Likewise, the mean (+/- S.E.) red blood cell carnitine concentration (RBC[C]) was significantly higher in preterm than in term neonates (0.24 +/- 0.02 versus 0.14 +/- 0.01 nmole/mg Hgb; P less than 0.001). Both PL[C] and RBC[C] were particularly elevated in extremely immature neonates (less than or equal to 33 wk gestation). Linear regression analysis showed a significantly negative correlation between PL[C] and gestational age (r = -0.332; P less than 0.001), and between RBC[C] and gestational age (r = -0.531; P less than 0.001). Approximately 72.2 +/- 1.1% (mean +/- S.E.) of carnitine in blood was estimated to be contained in the RBC, and 27.8 +/- 1.1% (mean +/- S.E.) of carnitine was estimated to be in the plasma.