Recently El-Bayoumy and coworkers have reported that 1,4-phenylene-bis(methylene)selenocyanate (p-XSC) was very effective in inhibiting 7,12-dimethylbenz(a)anthracene (DMBA)-induced mammary carcinogenesis and adduct formation during the initiation phase (Cancer Res., 52, 2402-2407, 1992). Furthermore, this compound was found to be well tolerated by rats at high doses. The present study was designed to extend these earlier observations by investigating the response to lower levels of p-XSC given either before or after DMBA administration. At a level of 15 p.p.m. Se, p-XSC suppressed total mammary tumor yield by 80% and 52% in the initiation phase and post-initiation phase, respectively. A dose-response effect was evident in the range 5-15 p.p.m. Se. When p-XSC was given at a level of 5 p.p.m. Se during the entire course of the experimental period, total tumor yield was reduced by half. This dose is about 4 x less than the maximum tolerable dose (MTD). Other selenocyanate analogs were also examined in an attempt to obtain information on their respective chemopreventive index, which is calculated as the ratio of MTD to the effective dose which produces approximately a 50% inhibition in total tumor yield (ED50). The reagents studied included potassium selenocyanate, methyl selenocyanate and benzyl selenocyanate, as well as sodium selenite (reference compound). Compared to p-XSC, which has a chemopreventive index of 4.0, the other four compounds have a lower index ranging from 1.3 for sodium selenite and potassium selenocyanate to 2.0 for methyl selenocyanate and 2.5 for benzyl selenocyanate. A high chemopreventive index signifies that a compound is well tolerated at doses required for cancer suppression. The last component of the present study involved the repletion assay of liver glutathione peroxidase in selenium-deficient rats as a biomarker to estimate the metabolizability of the above selenium compounds. The bioavailability data suggest that the selenium from p-XSC is not as efficiently incorporated into glutathione peroxidase as the selenium from selenite or the other selenocyanate analogs. Currently, we are working under the hypothesis that the chemical structure of the RSeCN compound could affect activity per se and also influence the rate of release of selenium from the parent compound, thereby impacting on the anticarcinogenic efficacy, tolerance and bioavailability of the compound.