In a preliminary study, 20 women in labour received ranitidine 50 mg intravenously. No significant changes were seen in the height, frequency or amplitude of uterine contractions or in fetal heart rate or pattern. No neonatal problems attributable to ranitidine were found. Ranitidine crossed the placenta, the mean fetal-maternal ratio being 0.9. Levels in the infants 12 hours following delivery were all very low. Ranitidine 150 mg orally was given to 80 healthy women undergoing elective Caesarean section at varying times from 75 to 510 minutes pre-operatively. From 2 to 6 hours following ingestion, the pH of gastric contents was greater than 2.5 in all but one patient. The mean volume aspirated (8 ml) was significantly lower than in a control group receiving magnesium trisilicate (mean volume 30 ml). Neonatal assessment included Apgar scoring, neurobehavioural examination, feeding progress, measurement of acidity and culture of gastric aspirates. No significant differences between groups were found. Blood levels indicated that the oral drug is readily absorbed by the parturient and that a smaller proportion is transferred to the fetus, mean fetal-maternal ratio at delivery being 0.38.