1 The metabolism of morphine was studied in 12 children and nine premature neonates on a continuous infusion of morphine (10-360 ,g kg-' h-1). 2 The mean plasma clearance of morphine was significantly higher in children than neonates (25.7 and 4.7 ml min-' kg-', respectively) (P < 0.01). 3 All the neonates and children had detectable concentrations of morphine-3-glucuronide (M3G) in plasma. All the children and five neonates had detectable concentrations of morphine-6-glucuronide (M6G) in plasma or urine. 4 The M3G/morphine ratios in plasma and urine, and M6G/morphine ratios in urine were significantly higher in children than neonates (P < 0.01), suggesting that morphine glucuronidation capacity is enhanced after the neonatal period. 5There was no difference in the M3G/M6G ratio in children and neonates, indicating a parallel development of both glucuronidation pathways.