The effects on mature newborn have been compared at 0.5, 4, 8 12 24 and 48 hr after birth, of maternally administered epidural bupivacaine (11 babies) or pethidine (18 babies) or pethidine reversed by naloxone administered intramuscularly to the newborn (15 babies). Bupivacaine (mean dose 130 mg) had less effect that pethidine (mean dose 183.3 mg) on alveolar carbon dioxide tension (PACO2) at 0.5 hr after birth, but had a similar effect to pethidine on feeding, elicited reflexes and produced more depression of muscle tone up to 48 hr. Bupivacaine had more effect on PACO2 feeding measures, elicited reflexes and muscle tone at almost all examination periods than pethidine (mean dose 157.0 mg) reversed by naloxone (200 micrograms intramuscularly). Except at delivery, the effects of bupivacaine or pethidine on respiration and feeding up to 48 hr after birth were similar. There were more signs of depression with both drugs than when pethidine had been reversed by naloxone.