We determined the ranitidine dosage necessary to maintain gastric pH at or above 4 in 40 critically ill children. The patients were divided into four groups of ten patients each. They were treated with ranitidine in the following dosages: a) 2 mg/kg by NG tube every 12 h; b) 4 mg/kg by NG tube every 12 h; c) 0.75 mg/kg iv every 6 h; d) 1.5 mg/kg iv every 6 h. The fourth group had a higher median pH than the other groups, in spite of also having the highest risk of acute gastric mucosal damage (AGMD). Eight (80%) of ten patients in the fourth group had a pH greater than or equal to 4 or more than 80% of the study period. We recommend 1.5 mg/kg iv every 6 h for gastric acid inhibition in AGMD prophylaxis in children.