A part of caseinphosphopeptides (CPP) formed during the digestion of casein in the small intestine of rats fed casein was not hydrolyzed in the digestive tract, but was excreted into the feces. Amino acid compositions of CPP fraction of feces for wk 1 and 2 were almost identical. The residual CPP in the feces expressed by the rate of bound phosphoserine in the CPP fraction of feces to bound phosphoserine ingested (phosphoserine-CPP/phosphoserine-ingested rate) in the ileum was the highest 4 h after the start of feeding of casein but decreased significantly after 10 h. No significant difference was observed in the rats of contents in jejunum, cecum, and colon between 4 h and 10 h after the start of feeding. No significant difference was observed in the phosphoserine-CPP/phosphoserine-ingested rate in contents of any part of the digestive tract between 50% casein and 50% CPP I (a commercial CPP product with nearly the same amino acid composition as that of casein) 4 h and 10 h after the start of feeding, except for the cecum 4 h after the start of feeding. No significant difference in phosphoserine-CPP/phosphoserine-ingested rate was observed in the contents of any part of digestive tract between the groups fed 50% casein and 5% CPP III +45% soybean protein isolate (SPI) 4 h or 10 h after the start of feeding (CPP III is a commercial CPP product containing nearly 8 times the concentration of phosphoserine as CPP I).(ABSTRACT TRUNCATED AT 250 WORDS)