A measuring method sensitive to prolyl endopeptidase (EC 3.4.21.26, PEP) activity using native peptides (Arg-vasopressin or substance P) as substrates was established. The investigation of three different derivatization reagents, which had been developed for an amino acid analysis, demonstrated that 4-fluoro-7-nitrobenzo-2-oxa-1,3-diazole (NBDF) was the most suitable for the detection of Arg-Gly-NH2, which was released from Arg-vasopressin by PEP. Arg-Gly-NH2 was reacted with NBDF at 65 degrees C for 5 min at pH 7.6 and the reaction mixture was analysed by HPLC on a reverse-phase column by monitoring the fluorescence intensity. The detection limit was 1 picomol per injection and the linear standard calibration curve could be constructed in the range of 1 to 100 picomol per injection with a 3.0% relative standard deviation. This sensitive detection method for peptide was applied to the measurement of PEP activity using Arg-vasopressin as a substrate and 1 x 10(-3) unit of PEP activity was detectable. This method was also applicable to the measurement of PEP activity using substance P as a substrate by detecting the derivative of its fragment peptide (Arg-Pro-Lys-Pro).