The inner layer of the vitelline membrane of avian oocyte is equivalent to the zona pellucida of mammalian oocytes or to the vitelline envelope of amphibian oocytes. One of the two major glycoproteins in the inner layer of quail vitelline membrane, formerly called 33-kDa glycoprotein, is homologous to mammalian ZPC, one of the components of zona pellucida. Quail ZPC is found to have different mobilities on SDS-polyacrylamide gel electrophoresis depending on whether it is obtained from the preovulatory follicle or from the laid eggs. In order to characterize the progressive changes in the molecular size of quail ZPC during the oviductal transport, the inner layer isolated from the follicle was incubated in vivo in various regions of the oviduct and subjected to Western blot analysis with anti-quail ZPC antiserum. The quail ZPC of the inner layer incubated in infundibulum reduced its apparent molecular weight, exhibiting the same electrophoretic mobility as that of laid eggs. The similar reduction in molecular weight was observed after the in vitro incubation of the inner layer with the extracts of infundibulum. From the comparison of the N-terminal amino acid sequences, it was found that the first 26 residues of the quail ZPC in follicular oocytes are missing from the ZPC of laid eggs. In addition, lectin blot analysis suggested the modification of oligosaccharide chains during the oviductal transport. These results represent the first description in the avian oviduct of the presence of protease, which is similar to oviductin, a trypsin-like protease involved in the hydrolysis of a major component of the vitelline envelope of amphibian oocytes. Mol. Reprod. Dev. 55:175-181, 2000.