1. A technique is described for perfusing the isolated cat pancreas with saline solutions.2. Single doses of secretin, although present in the perfusate for only a short time, caused a prolonged flow of pancreatic juice.3. In response to continuous secretin infusion, the preparation secreted for up to 6 hr a juice which was similar to that obtained in vivo, with the exception that the bicarbonate concentration decreased and the chloride concentration increased with time, even when the rate of secretion remained constant.4. The osmolalities of perfusate and secretion were identical over a range of 450 m-osmoles/kg, but the electrolyte concentration of the secretion was always slightly higher than that of the perfusate. Variations from perfusate isosmolality produced inverse changes in the secretion rate, over the range from 600 m-osmoles/kg, at which secretion ceased, to 150 m-osmoles/kg, at which the rate was highest. At perfusate osmolalities below 150 m-osmoles/kg secretion rapidly declined.5. Reduction in perfusate sodium chloride concentration, isosmolality being maintained with sucrose, caused a fall in secretion rate, but the sodium concentration of the juice remained constant until perfusate sodium concentration was reduced to about 70 m-equiv/l. Below this level it declined and sucrose was detected in the juice in quantities almost sufficient to account for the equiosmolality of juice and perfusate.6. Two hypotheses about the mechanism of water and electrolyte secretion by the pancreas are presented.