The effects of a single dose of 5 mg.kg-1 of ketamine administered intravenously to 10 critically ill preterm infants prior to epicutaneo-caval catheterization were analyzed using pulsed-wave Doppler ultrasound. The infants weighed between 670 and 1,885 g and their gestational ages ranged from 26 to 33 weeks. Arterial pressure (MAP), cardiac output (CO), transcutaneous oxygen pressure (TcPO2), transcutaneous carbon dioxide pressure (TcPCO2), end-diastolic velocity (EDV), peak systolic velocity (PSV), mean arterial velocity (MAV) of the cerebral anterior artery as well as Pourcelot's resistance index (PRI) were measured before and after injection of the drug. We observed a significant decrease in arterial pressure at 2 min after injection while heart rate and CO did not vary significantly. TcPO2 and TcPCO2, also remained unchanged throughout the period of measurement. EDV, PSV, and MAV did not vary significantly nor did PRI. As this drug provides major comfort to the baby during painful procedures and considerably facilitates difficult thin vessel catheterization, we believe that it may be used in such conditions.