Our previous study using cDNA microarray showed that differentially expressed genes among gastric cancer cells involved in peritoneal dissemination could be positively identified. One of these genes, which is overexpressed, is inositol 1,4,5-trisphosphate receptor type 3 (IP3R3). IP3R3 is responsible for the intracellular Ca2+ release channel and for mobilizing stored Ca2+. Three different receptor types have been molecularly cloned and their genes have been classified into a family. However, the role of the IP3 signaling pathway in the peritoneal dissemination of gastric cancer is still unclear. In the study presented here, the IP3R3 is showed to be overexpressed in gastric cancer cell lines established from malignant ascites, but weakly expressed in a gastric cancer cell line established from primary tumor as well as normal gastric epithelial cells. IP3R1 and 2 are only weakly or not expressed in these cells. The antagonist of IP3R, 2APB, inhibited cell proliferation and induced apoptosis in gastric cancer cells from malignant ascites at concentrations of 100 nM to 100 microM in a dose-dependent manner. On the other hand, 2APB showed a weak effect on other gastric cancer cells established from primary tumors (SNU1), lymph node metastases or liver metastases (MKN1 or 74), methothelial cell lines Met5A and myeloid leukemia HL60 cells. This suggests that this inhibitory effect depends on the level of IP3R3 expression. As cells which express IP3R3 mRNA (i.e. pancreas ascinar cells) are known to have a secretory function in which IP3/Ca2+ signaling has been shown to be involved, IP3R3 may be a prerequisite for secretion of an enzyme, such as protease, in gastric cancer cells. These results indicate that IP3R3 may be specifically involved in gastric cancer peritoneal dissemination and that IP3R3 may be a molecular target of the peritoneal dissemination of gastric cancer. Its antagonist, 2APB, may thus be useful for the specific treatment of peritoneal dissemination of gastric cancer.