A diagnostic agent for pancreatic exocrine function comprising a dipeptide represented by the following formula (I): X.sub.1--R.sub.1--Y.sub.1 (I) or a pharmaceutically acceptable salt thereof, wherein X.sub.1 is a protecting group, R.sub.1 is a phenylalanine, glutamine, valine, tyrosine, methionine, serine or threonine residue, and Y1 is a .sup.13C-labeled alanine molecule optionally having a protecting group.