To provide a rational basis for pancreatic enzyme replacement therapy, we evaluated, in six patients with advanced pancreatic insufficiency, the effects of various treatment regimens on fecal fat and nitrogen balance and on duodenal recovery of ingested pancreatic enzymes after a solid test meal. The combination of cimetidine (an H2-receptor antagonist) and pancreatin, each given by mouth, produced significantly higher postprandial duodenal recoveries and concentrations of trypsin and lipase (P less than 0.05). Steatorrhea was reduced in all patients and abolished in four of the six. In the dosages used, neither enteric-coated enzymes nor supplemental neutralizing antacids were more effective than pancreatin alone in decreasing steatorrhea or improving duodenal enzyme delivery. Cimetidine may be a useful adjunct to oral pancreatic extract therapy in some patients with severe pancreatic insufficiency who fail to respond to pancreatic enzyme replacement alone.