Aluminum hydroxide gel delays gastric emptying in rats and man. This effect of aluminum hydroxide gel varies with the concentration of aluminum in solution in the stomach when pH, osmolarity, and anion content are held constant. Because aluminum solubility drops as pH is raised, those antacids which neutralize more effectively than aluminum hydroxide gel alone and which contain "nonreactive" aluminum hydroxide result in lower aluminum concentration and do not affect the rate of gastric emptying in animals or in man. By using [51Cr]sodium chromate and a gamma camera technique, half-emptying time in 6 subjects with no gastrointestinal disease was shown to be prolonged from 13.1 min after water ingestion to 48.0 min after three hourly doses of aluminum hydroxide gel. Conventional nonabsorbable markers, including phenol red, were found to be of limited use for studying gastrointestinal function in the presence of antacid gels, most of which adsorb dyes.