Aim-To investigate the pharmacokinetics, metabolism, and dose-response relation of a single rectal dose of paracetamol in preterm infants in two diVerent age groups. Methods-Preterm infants stratified by gestational age groups 28-32 weeks (group 1) and 32-36 weeks (group 2) undergoing painful procedures were included in this study. Pain was assessed using a modified facies pain score. Results-Twenty one infants in group 1 and seven in group 2 were given a single rectal dose of 20 mg/kg body weight. Therapeutic concentrations were reached in 16/21 and 1/7 infants in groups 1 and 2, respectively. Peak serum concentrations were significantly higher in group 1. Median time to reach peak concentrations was similar in the two groups. As serum concentration was still in the therapeutic range for some infants in group 1, elimination half life (T 1 ⁄2) could not be determined in all infants: T 1 ⁄2 was 11.0 ± 5.7 in 11 infants in group 1 and 4.8 ± 1.2 hours in group 2. Urinary excretion was mainly as paracetamol sulphate. The glucuronide:sulphate ratio was 0.12 ± 0.09 (group 1) and 0.28 ± 0.35 (group 2). The pain score did not correlate with therapeutic concentrations. Conclusions-A 20 mg/kg single dose of paracetamol can be safely given to preterm infants in whom sulphation is the major pathway of excretion. Multiple doses in 28-32 week old neonates would require an interval of more than 8 hours to prevent progressively increasing serum concentrations. (Arch Dis Child Fetal Neonatal Ed 1999;80:F59-F63)