Capsaicin (CAP) has been known to inhibit some tumor development in vivo (J.J. Jang, S.H. Kim, T.K. Yun, Inhibitory effect of capsaicin on mouse lung tumor development, in vivo, J. Korean Med. Sci. 3 (1989) 49-53; J.J. Jang, K.J. Cho, Y.S. Lee, J.H. Bae, Different modifying responses of capsaicin in a wide-spectrum initiation model of F344 rat, J. Korean Med. 6 (1991) 31-36) [1,2] even though its mechanism of action is not well understood. The objectives of this study were to examine the effect of CAP on expression of tumor forming-related genes in a Korean stomach tumor cell, SNU-1. We used slot blot hybridization to investigate its effect on a wide spectrum of proto-oncogenes. It was found that CAP enhanced the transcripts of two proto-oncogenes (c-myc and c-Ha-ras) and tumor suppressor gene p53. While a low concentration of CAP (0.01 microM) did not significantly increase the level of p53 transcript in SNU-1, it did increase it by a factor of 3.5 at a 10 microM dose of CAP. Consequently, SNU-1 cells are sensitive to CAP in the overexpression of tumor suppressor gene, p53 and proto-oncogenes, c-myc and c-Ha-ras, but not those of c-erbB-2, c-jun and bcl-2 genes. Both cell death and DNA fragmentation were shown in SNU-1 cells with treatment of CAP. Our results suggest that CAP induces apoptotic cell death in human gastric cancer cells (SNU-1) in vitro which may be possibly mediated by the overexpression of p53 and/or c-myc genes. Because cell suicide is arguably the most potent natural defense against cancer, the correlation between the induction of apoptosis and the change of tumor forming-related gene expression after CAP treatment should be further studied in detail.