Pancreatic secretion in response to subcutaneously injected secretin was studied in three dogs with chronic pancreatic and gastric fistulas. Three solvents: saline, gelatin, carboxymethylcellulose (CMC); two volumes of solvent: 2 and 6 ml; and two doses of secretin: 75 and 150 clinical units were tested. Gelatin was more effective than CMC in prolonging the action of secretin; the duration of the plateau of secretion was about twice as long with gelatin as with saline. Increasing the volume of solvent from 2 to 6 ml also increased the total duration of secretion and total HCO-3 output. It is concluded that gelatin is a suitable vehicle for prolonging the action of secretin.