The effects of 20 mg of famotidine or placebo on secretion of gastric juice and gastric acid were studied in ten healthy subjects in a randomized, crossover study. Gastric juice was aspirated and collected at hourly intervals for 24 hours after oral administration, and acid output was calculated based on gastric juice output and acid concentration. Secretion of gastric juice and acid output were lower after famotidine was administered than after placebo; over the 12-hour post-administration period, the hourly acid concentrations were significantly lower after famotidine than after placebo. During the 12 hours after famotidine or placebo, gastric juice output was 180.7 +/- 32.1 ml (mean +/- SE) in the placebo group and 64.7 +/- 9.1 ml in the famotidine group (P less than 0.01); acid concentrations were 49.3 +/- 3.8 mmol/L in the placebo group and 9.6 +/- 3.1 mmol/L in the famotidine group (P less than 0.01); acid output was 8.89 +/- 1.59 mmol in the placebo group and 0.55 +/- 0.17 mmol in the famotidine group (P less than 0.01). These results support the effectiveness of a 12 hour or possibly a 24-hour dosing interval for famotidine.