The pharmacokinetics of morphine was studied in 27 infants receiving a single intravenous dose of 0.1 mg/kg morphine after surgery (n = 23) or during mechanical ventilation (n = 4). The pharmacokinetics of morphine varied greatly between the subjects, especially in the neonates. The clearance and half-life varied distinctly with postnatal age. The mean (+/- SD) half-life was 8.1 +/- 8.1 h in 10 neonates younger than 1 week, 5.4 +/- 3.4 h in 10 infants aged from 1 week to 2 months and 2.6 +/- 1.7 h in 7 infants aged from 2 to 6 months. Mean clearance increased significantly with age, being 8.7 +/- 5.8 ml/min/kg in the youngest age group, 11.9 +/- 5.1 ml/min/kg in those aged 1 week to 2 months and 28.0 +/- 8.9 ml/min/kg in those aged 2-6 months, and was also significantly lower in the critically ill infants. The clearance and half-life of morphine begin to approach adult values after the age of 1 month, but great individual variability exists even after that. In order to reduce the risk of overdosing or underdosing, the dose of morphine should be titrated individually.