Apoptois is an important determinant in the sensitivity to chemotherapeutic agents in gastric cancer cells. In this study, we examined whether the introduction of the bax gene into MKN45 gastric cancer cells could enhance the sensitivity to chemotherapeutic agents in association with apoptosis. Apoptosis in the bax-transfected gastric cancer cells was enhanced following the treatment of various chemotherapeutic agents including adriamycin (ADM), cisplatin (CDDP), etoposide (VP-16) and taxotere (TXT) as compared to those of neo gene-transfected cells. The enhancement of apoptosis was coincident with the increase of sensitivity in the ratio of IC50 value, that was 1.3-fold in ADM, 4.4-fold in CDDP, 4.6-fold in VP-16 and 2.5-fold in TXT, respectively. Further, the enhancement of apoptosis in the bax-transfected gastric cancer cells was associated with the activation of c-Jun N-terminal kinase 1 (JNK 1) and caspase 3 (CPP32). The increases of sensitivities to these agents in the bax-transfected cells were also demonstrated in in vivo experiments using the tumor cells transplanted into nude mice. The tumor growth in the bax-transfected cells was significantly suppressed following the treatment of CDDP or VP-16 compared to that of neo-transfected cells (p < 0.05). These results indicated that, the bax gene might play a critical role in determination of sensitivity to chemotherapeutic agent in gastric cancer cells in vivo, and that the activation of JNK 1 and CPP32 might be involved in the signal transduction pathways leading to apoptosis.