Results of epidemiologic and experimental studies suggest that selenium can inhibit the development of tumors. In rats, the administration of selenium decreases the incidence of carcinogen-induced colon tumors; the inhibition is greater in the proximal colon that in the distal colon. We investigated the distribution of selenium in the different segments of rat colon and determined the uptake of selenium in the mucosa and in the muscle layers of each segment. The colon was perfused before removal of the segments to ensure complete removal of blood-borne selenium. We found that the concentration of selenium was greater in the proximal colon than in the distal colon and that within each segment the uptake was higher in mucosa than in muscle. In addition, we determined the level of selenium in blood, serum, and liver at different times after the administration of various doses of selenium. Though the mechanism by which selenium prevents tumor development is unknown, the data indicate a correlation between the uptake of selenium in different segments of colon and inhibition of tumorigenesis.