Morphine pharmacokinetics and pharmacodynamics (analgesia and sedation) were evaluated after continuous intravenous infusion of morphine in 19 neonates, both preterm and term, whose lungs were ventilated to relieve respiratory distress. Elimination half-life, total plasma clearance, and volume of distribution (mean +/- SD) were 9.6 +/- 3.0 hours, 2.55 +/- 1.65 ml/min/kg (area analysis) or 2.09 +/- 1.19 ml/min/kg (steady-state data), and 2.05 +/- 1.05 L/kg, respectively, and were not significantly different in preterm and term neonates. In neonates with adverse effects of morphine, the plasma clearance was decreased twofold. Mean morphine concentration required to produce adequate sedation in 50% of patients was found to be 125 ng/ml, but concentrations above 300 ng/ml may be associated with adverse effects of morphine. Morphine-6-glucuronide was not detected in the plasma of any neonate, which may explain why neonates require high plasma concentrations of unchanged morphine for sedation.