The gastrojet, a closed loop pH feedback infusion pump capable of maintaining intragastric pH at a target value by infusing H2 blockers at variable rates, was used to assess factors influencing the quantity of famotidine required to maintain intragastric pH above 4 for 24 hours in 34 fed patients with duodenal ulcers. The following factors were considered: sex, age, duration of the disease, previous bleeding, previous poor response to H2 blockers (ulcer unhealed at six weeks, or recurrence within three months during maintenance treatment), activity of the ulcer disease, smoking habits, cirrhosis. The patients had taken no antisecretory drugs for the 15 days before the study. Two standardised meals were given during the study period (from 1000 to 1000). Fifty ml of famotidine (4 mg/ml) was loaded into infusion bags and the pump was programmed to deliver the drug intravenously at 11 rates varying from 0 to 40 microliters/min. The target pH was 4. Mean famotidine use was 111 mg (range 33 to 200), the 23 hour median pH was 5.3, and the mean time during which pH was above 4 was 75.4%. There was a negative correlation (p < 0.001) between famotidine delivery and the inhibition of gastric acidity. Statistical analysis showed that only cirrhosis significantly influenced drug delivery, median pH, and the time during which pH was above 4. Mean drug delivery in the cirrhotic and non-cirrhotic patients was 135 v 97 mg (p < 0.04), 23 hour median pH was 4.7 v 5.6 (p < 0.01), and the mean time at pH > 4 was 65.9 v 81.6% (p < 0.01). There were large interindividual variations in famotidine requirements, but not only cirrhosis was predictive of dose requirement. These results suggest that the appropriate amount of famotidine to treat duodenal ulcer in cirrhotic patients is probably higher than the usually recommended dose.