The effects on gastric pH of various forms of administration of antacids have been studied in healthy volunteers. Gastric pH was recorded using a radiotelemetric technique (Heidelberg Capsules). In one series of experiments the Heidelberg Capsules were compared with a conventional aspiration method of measuring gastric pHqt lower pH-levels (pH smaller than 2) the Heidelberg Capsules recorded lower values than what was found with the aspiration technique. However, when the pH-response to an antacid dose was studied the two techniques gave concordant results. In another series of experiments, four antacids with the same acid-neutralizing capacity in vitro were compared in fasting subjects, using a crossover design. A liquid preparation and a chewable tablet gave almost equal results with a short latency time and a duration of action of more than one hour (mean 69 minutes and 82 minutes, respectively, with pH above the basal level). Two other tablet antacids designed to be swallowed whole gave less prompt effects and a shorter duration of action (mean 45 minutes for both, with pH above the basal level).