MMP-7 is a matrix-degrading enzyme that is mainly produced from cancer cells, and has a great role in the invasion and metastasis of cancer. We have established a highly metastatic cell line (MKN-45-P) on the peritoneum of nude mice from MKN-45 by repeated intraperitoneal inoculation of intraperitoneal free cancer cells. By the precise screening of metastasis-related genes using reverse transcriptase-polymerase chain reaction (RT-PCR), MKN-45-P characteristically expressed more MMP-7 than the original cell line of MKN-45. In this study, we studied the effects of antisense oligonucleotides complementary to exon 3 of MMP-7 mRNA on the expression of MMP-7 and metastatic potential of MKN-45-P by using in vitro and in vivo experiments. RT-PCR and western blot analysis demonstrated that 10 microM antisense oligonucleotides suppressed MMP-7 expression at both the mRNA level (84%) and protein level (56%). Antisense oligonucleotides, specific for MMP-7 suppressed invasion by MKN-45-P cells without influencing proliferation. On the other hand, scrambling sequence control oligonucleotides did not show any inhibitory effects. In addition, survival of MKN-45-P bearing mice, which had been treated for 48 hrs with antisense oligonucleotides before intraperitoneal injection, was significantly better than that of control mice. In contrast, control oligonucleotides did not influence the survival of mice with the peritoneal dissemination model. These results strongly suggest that MMP-7 may have a great role in the formation of peritoneal dissemination in gastric cancer, and the molecular control of MMP-7 using antisense oligonucleotides may be a hopeful treatment modality for peritoneal dissemination.