To examine a direct involvement of genotoxic effects of estrogens in the initiation of hormonal carcinogenesis, the abilities of 17beta-estradiol (E2) and 8 of its metabolites to induce cellular transformation and genetic effects were studied using the Syrian hamster embryo (SHE) cell model. Treatment with E2, estrone (E1), 2-hydroxyestrone (2-OHE1), 4-hydroxyestrone (4-OHE1), 2-methoxyestrone (2-MeOE1), 16alpha-hydroxyestrone (16alpha-OHE1), 2-hydroxyestradiol (2-OHE2), 4-hydroxyestradiol (4-OHE2) or estriol (E3) for I to 3 days inhibited SHE cell growth in a concentration-dependent manner. Concentration-dependent increases in the frequency of morphological transformation in SHE cells were exhibited by treatment for 48 hr with each of all estrogens examined, except for E3. The transforming activities of the estrogens, determined by the induced transformation frequencies, were ranked as follows: 4-OHE1 > 2-OHE1 > 4-OHE2 > 2-OHE2 > or = E2 or E1 > 2-MeOE1 or 16alpha-OHE1 > E3. Somatic mutations in SHE cells at the Na+/K+ATPase and /or hprt loci were induced only when the cells were treated with 4-OHE1, 2-MeOE1 or 4-OHE2 for 48 hr. Some estrogen metabolites induced chromosome aberrations in SHE cells following treatment for 24 hr. The rank order of the clastogenic activities of the estrogens that induced chromosome aberrations was 4-OHE1 > 2-OHE1 or 4-OHE2 > 2-OHE2 > E1. Significant increases in the percentage of aneuploid cells in the near diploid range were exhibited in SHE cells treated for 48 hr or 72 hr with each of the estrogens, except for 4-OHE1 and E3. Our results indicate that the transforming activities of all estrogens tested correspond to at least one of the genotoxic effects by each estrogen, i.e., chromosome aberrations, aneuploidy or gene mutations, suggesting the possible involvement of genotoxicity in the initiation of estrogen-induced carcinogenesis.