The influence of cimetidine on the lower esophageal sphincter, the intragastric pH and the serum levels of immunoreactive gastrin was investigated in eight volunteers and compared with a control group. After oral application of 400 mg cimetidine, the intragastric pH rose from 1.7 ± 0.13 to 6.2 ± 0.4 and the lower esophageal sphincter pressure increased slightly but significantly from 15.6 ± 1.1 to 22.7 ± 1.8 mm Hg after 20 min and to 25.8 ± 2.0 after 30 min, respectively. At the same time the serum immunoreactive gastrin levels did not change. A direct effect of cimetidine on the lower esophageal sphincter or an indirect effect via gastric alkalinization is discu