Enzymatically synthesized oligo-L-methionine (OM), which consists of 5 to 12 polymerized L-methionine, is an insoluble, slowly digestible peptide. Digestibilities of diets with OM (3 g/kg diet) containing enzymatic hydrolysates of soybean protein isolate (SPI), large- and small-SPI peptides, were higher than those with OM added to an intact SPI diet at a low protein level (100 g/kg diet). The digestibility of OM was decreased when the content of these protein sources in diets was increased from a low level to a high level (200 g/kg diet). We examined the relation between the pancreatic protease secretion rate and the OM digestibility. Chymotrypsin secretion was strongly stimulated by feeding a low large-SPI peptide diet, followed by an SPI diet, and slightly enhanced by feeding a small-SPI peptide diet. The protease secretion was not correlated with OM digestibility. We also examined the inhibition of pancreatic digestion of OM by these protein sources. More than 20% of OM was digested in vitro by a low concentration of rat bile-pancreatic juice during a 1 hr incubation, which was evaluated by solubilization of a radiolabeled OM. The in vitro OM digestion was markedly inhibited by the addition of SPI and slightly inhibited by small-SPI peptides. The degree of inhibition by large-SPI peptide was between the values of SPI and small-SPI peptide. These protein sources inhibited the in vitro OM degradation in a dose-dependent manner. These results agree with in vivo OM digestibility. We conclude that the decreases in the inhibition of pancreatic digestion of OM improve OM digestibility more efficiently than the increase in the pancreatic protease secretion in rats fed SPI peptides.