In a double-blind, randomized study, six healthy women were given four different antacids containing aluminium, magnesium or calcium, for a period of seven days, respectively. The antacids investigated were Trigastril 50 Del, Maaloxan Suspension, Kompensan-S forte Suspension and Solugastril 50 Gel. All the antacids tested led to a significant increase in the stool frequency and accelerated stool transit time. The sodium-containing agent produced a significantly greater acceleration as compared with the other antacids. No changes in the sodium, potassium or calcium blood levels were seen prior to and after administration of the various antacids. All preparations reduced phosphate concentrations (significant only with Solugastril an Trigastril). In the case of Maaloxan with a high content of magnesium, the magnesium concentration was increased mildly, but significantly. Irrespective of the dose of aluminium employed, all the antacids resulted in a significant increase in aluminium concentration of between 23 and 36 micrograms/l. All the antacids significantly reduced the excretion of phosphate in the urine. The preparation with a high level of magnesium produced the greatest excretion of magnesium, the preparation containing calcium the greatest excretion of calcium, in 24-hour urine. The results show that the effects on the weight of the stools, the transit time, the consistency of the stools, and on the mineral balance, depends upon the composition of the respective antacid.