A randomized double-blind study compared the effects of equi-analgesic doses of maternally administered meptazinol (1.5 mg/kg) and pethidine (1.5 mg/kg) on neonatal acid-base status. Heel-prick samples were taken for assessment of acid-base status at 10 and 60 min after delivery. Maternal antenatal history, details of labour and neonatal status at delivery were also recorded. Meptazinol produced less neonatal respiratory depression than pethidine: the mean 10 min acid-base data from 16 infants whose mothers received pethidine were indicative of a respiratory acidosis (pH 7.13, SD 0.08, PCO2, 9.11, SD 2.2 kPa; standard bicarbonate 22.3, SD 3.1 mmol/l). This was not evident in the mean acid-base data from 16 infants whose mothers received meptazinol (pH 7.23, SD 0.07; PCO2 6.83, SD 1.6 kPa; standard bicarbonate 20.9, SD 4.2 mmol/l). The mean pH and PCO2 in the two treatment groups were significantly different (P less than 0.002) at 10 min but not at 60 min after delivery.