We studied the effect of gastric versus jejunal tube feedings on gastric pH and evaluated the acid-inhibiting effects of continuous gastric and jejunal infusions of cimetidine. pH was monitored by an intragastric pH probe in 19 gastrostomy and 13 jejunostomy patients during fasting, continuous infusions of Osmolyte, cimetidine, and simultaneously Osmolyte and cimetidine. Gastric Osmolyte increased fasting pH from a mean of 1.32 to 2.78 (P less than 0.01), while jejunal Osmolyte did not (pH 1.02). Continuous gastric infusion of cimetidine maintained a high gastric pH (5.06) in 17 of 19 patients. Jejunal cimetidine also achieved therapeutic serum levels and raised gastric pH to 4.81 in nine of 13 patients. We conclude that the persistently low gastric pH during jejunostomy tube feeding may play a major role in the upper gastrointestinal bleeding previously observed in such patients and that continuous gastric and jejunal cimetidine infusions effectively raise and sustain a high gastric pH.