Lower oesophageal sphincter pressure and fasting plasma gastrin and progesterone were measured in 31 women in the last trimester of pregnancy and in 10 healthy female control subjects. Eighteen of the pregnant women suffered from heartburn but 13 did not. All of the control subjects and 10 women from each of the two pregnant groups were tested for gastro--oesophageal reflux by direct measurement of intraluminal pH. The mean barrier pressure of the lower oesophageal sphincter was lower in both groups of pregnant women than in the controls (P less than 0-05) and the mean barrier pressure of the women with heartburn was lower than that of the pregnant women without heartburn, though this difference did not reach statistical significance. Eight of 10 of the pregnant women with heartburn had moderate or severe reflux, and3 of 10 of the pregnant women without heartburn also had moderate or severe reflux. Most women who reflux have heartburn, nevertheless, some asymptomatic women also reflux, and therefore all pregnant women must be considerered at risk from Mendelson's syndrome if subjected to a general anaesthetic for an emergency obstetric procedure.