A diagnostic agent for pancreatic exocrine function comprising an amino acid or a peptide containing at least one .sup.13C or .sup.14C atom, or a pharmaceutically acceptable salt thereof other than Bz-Tyr-.sup.13C-PABA is provided. A .sup.13C- or .sup.14C-labeled compound represented by the following formula (II): X.sub.2--R.sub.2--Y.sub.2--Z.sub.1 (II) or a salt thereof, wherein X.sub.2 is a hydrogen atom or a protecting group, R.sub.2 is a peptide of 2 to 5 amino acids, an amino acid or a single bond, Y.sub.2 is an amino acid, Z.sub.1 is an amino acid optionally having a protecting group, and at least one of the amino acids in R.sub.2, Y.sub.2 and Z.sub.1, or at least one of the protecting groups in X.sub.2 and Z.sub.1 when the protecting groups contain a carbon atom, is .sup.13C or .sup.14C-labeled.