Vitamin K prophylaxis usually is administered orally or intramuscularly, but in neonatal intensive care oral administration might not be feasible and intramuscular administration is not general practice in very small infants. No data are available about plasma levels after intravenous administration of vitamin K to neonates. Therefore, we investigated plasma levels in 18 infants: 14 preterms with a birthweight of 1785+/-648 g and 4 sick newborns with a birth-weight of 3167+/-510 g after administration of a single dose of 0.3+/-0.1 mg/kg phylloquinone (vitamin K(1)) (Konakion MM((R)), Roche) intravenously after birth. Blood was collected 22.9+/-18.4 hours after intravenous administration of vitamin K(1). In 10 neonates a second sample was obtained 111.8+/-49.1 hours after the first vitamin K(1) administration. Gas chromatography-mass spectrometry (GC-MS) was used as the method for determination of vitamin K(1). The measured plasma concentration after intravenous administration of vitamin K(1) was 191.3+/-102.6 ng vitamin K in the first sample /mL in the first sample and 98.7+/-75.2 ng vitamin K(1)/mL in the second samples. These results are similar to those described in newborns after oral administration of 3 mg vitamin K(1) and after intramuscular administration of 1.5 mg vitamin K(1). In conclusion, the recommendation of the producer to give 0.4 mg/kg of vitamin K intravenously to neonates, in whom oral or intramuscular administration is not feasible, seems to be rational.