We have consistently shown that several synthetic Organoselenium compounds are superior cancer chemopreventive agents and less toxic than selenite or certain naturally occurring selenoamino acids. 1,4-Phenylenebis(methylene)selenocyanate (p-XSC) is the lead Organoselenium compound in that it has been shown to be the most effective and the least toxic agent in several experimental cancer models. It is not known whether p-XSC or one of its metabolites is responsible for its chemopreventive efficacy. As an initial step, we synthesized one of its putative metabolites, i.e., the glutathione conjugate of p-XSC (p-XSe-SG), and determined its stability in the pH range from 2 to 8 and in the diet under normal feeding conditions. We also assessed its maximum tolerated dose and examined its chemopreventive efficacy against azoxymethane (AOM)-induced colon carcinogenesis in male F344 rats. p-XSe-SG proved to be very stable over the pH range tested. The maximum tolerated dose of p-XSe-SG determined in a 6-week subchronic toxicity study was found to be >210 ppm (>40 ppm selenium) when the compound was added to AIN-76A high-fat diet. To assess the efficacy of this agent in the postinitiation period of colon carcinogenesis, male F344 rats 6 weeks of age were fed the high-fat diet, and at beginning of weeks 7 and 8, all of the rats intended for carcinogen treatment were given AOM at a dose of 15 mg/kg body weight by s.c. injection. Two days after the carcinogen treatment, the groups of rats consuming the high-fat control diet began their respective high-fat experimental diet regimens with 0, 56, or 84 ppm p-XSe-SG (0.1, 10, and 15 ppm of selenium) supplementation. All animals continued on their respective diets for 38 weeks after the AOM-treatment and were then killed. Colon tumors were evaluated histologically using routine procedures and were also analyzed for cyclooxygenase (COX)-1 and COX-2 expression and enzymatic activities. The results indicate that p-XSeSG administered during the post-initiation stage significantly inhibited both the incidence (P < 0.05-0.01) and the multiplicity (P < 0.05-0.005) of AOM-induced colon adenocarcinomas. This agent also greatly suppressed the multiplicity (P < 0.01-0.001) of AOM-induced exophytic adenocarcinomas in a dose-dependent manner. Feeding of 56 or 84 ppm p-XSe-SG in the diet significantly suppressed total COX activity (P < 0.02 to -0.01) and COX-2 specific activity (P < 0.005-0.0005) but had minimal effect on the protein expression levels of COX-1 and COX-2. These results suggest that the newly developed synthetic Organoselenium compound, p-XSe-SG, is stable in the diet and at wide pH ranges, inhibits colon carcinogenesis when administered during the postinitiation stage, and inhibits COX activity. Compared with previous efficacy studies and considering the toxicity associated with selenium, p-XSe-SG seems to be the least toxic Organoselenium chemopreventive agent thus far tested in the experimental colon carcinogenesis. Studies are in progress to delineate whether p-XSe-SG is also effective when administered during the progression stage of colon carcinogenesis.