To determine the effects of fentanyl in newborn and premature infants, we compared two groups of 20 newborn and premature babies under artificial ventilation for severe respiratory distress syndrome: a prospective group receiving fentanyl for analgesic and sedation and a historical control group, who did not receive fentanyl. Fentanyl serum levels during steady state were determined by radioimmunoassay. Average time of infusion was 86 +/- 47 h with a mean dosage of 0.68 +/- 0.24 micrograms/kg/h. The fentanyl group needed much less sedatives and catecholamines. Heart rate and blood pressure were not significantly changed by fentanyl. Meconium was excreted later, and higher values of bilirubin were reached earlier than in the control group. Although fentanyl proved to be helpful in the neonatal intensive care unit, the administration should remain under strict indication.