Ranitidine 150 mg orally was given every 6 hours to 909 women in labour, while a control group of 378 women received conventional alkali therapy. No differences in incidences of operative intervention, placental retention or post-partum haemorrhage were observed between groups. Gastric sampling during emergency anaesthesia revealed a pH less than 2.5 in four of 51 women who received ranitidine and in two of 31 women who received magnesium trisilicate. Gastric volumes were slightly lower (mean 83 ml) in the study group than in the control group (mean 122 ml). Absorption of ranitidine was greatly slowed following narcotic administration and gastric volume was significantly higher in those patients given narcotics in labour. Apgar scores were similar in both groups of infants, and babies whose mothers were given ranitidine showed no delay in achieving high gastric acidity and no increase in bacterial colonization of the gastro-intestinal tract. Low levels only of ranitidine were found in the blood of babies at 2-3 hours and approximately 12 hours after birth.