A genotoxicity evaluation of three commonly used estrogens-ethinyl estradiol, cyclotriol and cyclodiol was undertaken using short-term in vitro and in vivo assays. None of the drugs caused significantly increased or decreased number of His+ mutants to appear in the Ames Salmonella assay, either with or without S9 mix or in a modified host-mediated version of this assay. However, the clastogenic potential of these drugs became evident from the increased number or chromosome aberrations and sister chromatid exchanges (SCEs) induced by these drugs in human lymphocyte cultures both in the presence and absence of S9 mix. Increased frequencies of micronuclei and of sister chromatid exchanges in mice confirmed their clastogenic potential.