TPP-Ⅰ(tripeptidyl peptidase-Ⅰ) protein turnover was studied by observing the role of rat TPP-Ⅰ in neuromedin B(NMB) and other neuropeptides degradations in some physiological situations. The mixtures of rat TPP-Ⅰ with each of NMB, other neuropeptides and Ala-Ala-Phe-MCA were made respectively under the same conditions. The reaction was observed at different timeand monitored by means of high performance liquid chromatography(HPLC) and time-of-flight mass spectrometry(TOF-MS) in vitro. NMB was broken down at the same degree as Ala-Ala-Phe-MCA by rat TPP-Ⅰ and Gly-Asn-Leu was released within 16 h, but other neuropeptides were not digested within 24 h. TPP-Ⅰ is the predominant proteolytic enzyme responsible for the intracellular degradation of neuromedin B. NMB has recently been found to be a good natural substrate for rat lysosomal TPP-Ⅰ.