This experiment was designed to observe the effects of 3 ppm and 6 ppm selenium in water on preneoplastic lesions in liver and hepatocarcinogenesis induced by AFB1. In the group administered selenium, the number of hyperplastic foci and enzyme altered foci was much less than that in the AFB1 group. From the 52nd to 79th week, cancers were observed in 11 of 18 rats in the AFB1 group, while none was found in the selenium treated groups. These results reveal that selenium can inhibit the formation of hyperplastic foci and enzyme--altered foci as well as hepatocarcinogenesis induced by AFB1, but selenium can neither prevent the enlargement nor accelerate the regression of the foci already developed after administration of carcinogens. Additionally, toxic effect of selenium was found in 6 ppm selenium group; the number and size of foci were greater in 6 ppm selenium group than those in 3 ppm group. Therefore, adequate amount of selenium is probably the link to achieve better inhibitory effect.