The effects of selenium (Na2SeO3) on aflatoxin B1 (AFB1)-induced hepatic neoplasia were studied in the rat. Putative preneoplastic foci and nodules composed of basophilic, eosinophilic, and clear cells developed early. Basophilic foci were seen first; in the later stages basophilic and eosinophilic nodules predominated. At each stage the AFB1 + Se groups showed fewer and smaller foci and nodules than the AFB1 - Se group. The number of foci in the AFB1 + 3 ppm Se group and their mean area were smaller than those in the 6 ppm Se + AFB1 group. At the end of the experiment hepatocellular carcinoma (HCC) was found in 11/18 rats (61%) of the AFB1 - Se group. HCC was not found in either of the groups given AFB1 + Se. We conclude that Se had an inhibitory effect on the initiation and promotion stages of AFB1-induced preneoplastic foci and nodules. Se also prevented progression of these nodules to HCC even after cessation of AFB1 administration. The inhibitory effect of Se at 3 ppm was greater than at 6 ppm. The 6 ppm Se group also showed evidence of toxicity.