The antiestrogen tamoxifen is widely used for the treatment of breast cancer and more recently for the prevention of breast cancer. A concern over the use of tamoxifen as a chemopreventive agent is its carcinogenicity in rat liver, through a genotoxic mechanism involving alpha-hydroxylation, esterification, and DNA adduct formation, primarily by reaction with dG. In a recent study [Gamboa da Costa et al., Cancer Lett., 176, 37-45 (2002)], we demonstrated a significant increase in the mutant frequency in the lacI gene of Big Blue rats treated with tamoxifen, and a further increase in rats administered alpha-hydroxytamoxifen. In the present study, we have assessed mutation induction by tamoxifen and alpha-hydroxytamoxifen in the liver cII gene of Big Blue rats and have characterized the types of mutations induced by alpha-hydroxytamoxifen in the liver lacI and cII genes. The mutant frequencies in the liver cII gene were 80 +/- 13 x 10(-6) in the control, 112 +/- 13 x 10(-6) in the tamoxifen-treated group (P < 0.01 vs. control), and 942 +/- 114 x 10(-6) in the alpha-hydroxytamoxifen-treated animals (P < 0.001 vs. control; P < 0.001 vs. tamoxifen). Molecular analysis of the mutants indicated that the alpha-hydroxytamoxifen-induced mutational spectrum differed significantly from the control spectrum, but was very similar to the spectrum induced by tamoxifen for both the lacI and cII genes [Davies et al., ENVIRON: Mol. Mutagen., 28, 430-433 (1996); Davies et al., Carcinogenesis, 20, 1351-1356 (1999)]. G:C --> T:A transversion was the major type of mutation induced by alpha-hydroxytamoxifen and tamoxifen, while G:C --> A:T transition was the main type of mutation in the control. These results support the hypothesis that alpha-hydroxytamoxifen is a major proximate tamoxifen metabolite causing the initiation of tumors in the liver of rats treated with tamoxifen.