This study was designed to establish appropriate dosing requirements for intravenous use of famotidine, a new H₂-receptor antagonist, in pediatric patients. Eighteen children, 2 to 69 months of age (mean ±SD: 31±23 months), were treated with intravenously administered famotidine to prevent bleeding of the upper gastrointestinal tract. Continuous intragastric pH monitoring was carried out to ascertain the effectiveness of various intravenous doses. An initial dose of 0.4 mg/kg was given and repeated only after the intragastric pH had dropped to <4.0 for 2 hours. Increased doses were given (0.8, 1.2, and 1.6 mg/kg) if the previous dose did not raise the intragastric pH to >4.0 for ≥6 hours. Sixteen patients achieved an intragastric pH≥4.0 for ≥6 hours, 13 with a dose of 0.4 mg/kg, and three with a dose of 0.8 mg/kg. The mean duration of continuous pH>4.0 per dose was 9.0±7.5 hours. Ten patients recelved two or more intravenous doses of famotidine; the mean duration of pH>4.0 after the second dose was less than that after the first (14.9±8.0 hours vs 6.9±3.2 hours, p<0.01). We conclude that intravenous famotidine therapy raises gastric pH to>4.0 for approximately 9 hours in most children. Prolonged intravenous use of famotidine rapidly leads to a decreased duration of efficacy, necessitating intragastric pH monitoring to assess the effectiveness of treatment.