Exocrine pancreatic insufficiency in the dog has been assessed by the oral administration of the synthetic peptide N-benzoyl-L-tyrosyl-p-aminobenzoic acid (BT-PABA), a specific substrate for pancreatic chymotrypsin. The subsequent assay of PABA in either the plasma or the urine clearly differentiated control animals from those with exocrine pancreatic insufficiency (EPI), the results being unaffected by combination of this pancreatic function with a xylose absorption test. Possible interference with the specificity of the peptide test for the diagnosis of EPI was examined in six animals with small intestinal disease. In a group of four animals, with features resembling chronic tropical sprue in man, the results were comparable to those of the control group. In the fifth case, however, the results were indistinguishable from those of the EPI group, the estimation of sodium PABA absorption and the assay of proteolytic activity in the duodenal juice demonstrating that this was due to defective hydrolysis of the peptide. In the sixth case, diffuse intestinal lymphosarcoma and a marked villous atrophy were associated with an apparent reduction in the absorption of sodium PABA. However, although the plasma PABA concentrations following oral BT-PABA were subnormal, they were distinctly higher than those of the EPI group. These findings suggest that small intestinal abnormalities do not affect PABA absorption sufficiently to interfere with the specificity of the peptide test for the detection of severe EPI in the dog. This insufficiency may occasionally be secondary to small intestinal disease.