Summary.The combined treatment of cats with sulfapyridine and secretin is described. In conformance with other investigations, in which secretin was not used, the concentration of sulfapyridine in pancreatic juice was found to be just lower than that of blood, but an extremely high concentration of the drug was found in the bile. The drug is not diluted by the increased flow of juice either from pancreas or liver and tends to be independent of rate of flow. Blood concentration of sulfapyridine is more important. While there was no change in the percentage of free drug excreted in the pancreatic juice, 30 % of the free drug had been bound when excreted in the bile, indicating acetylation by the liver.