Lower esophageal sphincter pressure, basal gastric pH, fasting plasma gastrin, and plasma concentrations of estrone, estradiol, and progesterone were measured in pregnant volunteers at 12, 24, and 36 weeks of gestation, and again at 1 to 4 weeks postpartum. In addition, basal and pentagastrin-stimulated acid secretory responses at each time were measured. No differences in basal gastric pH, basal, and peak acid outputs were observed during pregnancy when compared to the postpartum values. In contrast, lower esophageal sphincter pressure was reduced at all times during pregnancy, reaching a nadir at 36 weeks. Postpartum lower esophageal pressures were normal. As expected, plasma concentrations of progesterone and both estrogens increased progressively during pregnancy. These data are consistent with earlier studies in women ingesting oral contraceptives. Moreover, they provide support for the thesis that the progressive increase in plasma progesterone alone or in combination with estrogens that occurs during pregnancy is responsible for the reduction of lower esophageal sphincter pressure which allows esophageal reflux to occur with the resultant development of symptomatic heartburn.