Recent studies suggest that there is a chemical perception of dietary fat in the oral cavity. A much greater response in the cephalic phase of pancreatic secretion is obtained by palatable taste stimuli. To examine the perception of dietary fat in the oral cavity, we determined whether or not oral stimulation with fat alters the cephalic phase of pancreatic enzyme secretion. A cannula was inserted into the common bile duct of rats to collect bile-pancreatic juice, and the animals were esophagostomized to prevent stimulation of the gastrointestinal tract with the test substances. The cephalic phase of pancreatic enzyme secretion was enhanced by oral stimulation with the long chain fatty acids, oleic acid, linoleic acid and linolenic acid, but was unaffected by the middle chain fatty acid, caprylic acid, and long chain fatty acid derivatives, methyl oleate and methyl linoleate. This selectivity is consistent with our previous findings that, in a short term two-bottle preference test, rats prefer long chain fatty acids to triglycerides, middle chain fatty acids or long chain fatty acid derivatives. These results suggest that long chain fatty acids trigger chemical reception in the oral cavity, and the length of the fatty acid chain and the presence or absence of carboxylate groups are related to recognition of fat.