Genotoxic evaluation of a commonly used progestogen, norethisterone acetate, was undertaken using a combination of short-term in vitro and in vivo assays. The clastogenic potentiality of norethisterone acetate was evident from the chromosome aberrations and sister chromatid exchanges induced both with and without S9 mix in cultured human lymphocytes and also from the increased frequency of micronuclei formation and sister chromatid exchanges in mice. However, in the Ames Salmonella assay, both with and without S9 mix and in host-mediated assay, norethisterone acetate was unable to cause any significant increase/decrease in the His+ revertants/plate.