In this study, antiproliferative effects of the selective estrogen receptor modulator Tamoxifen and the aromataseinhibitor letrozole (Femara) were evaluated and compared using the FM3A cell line, originating from a C3Hmouse mammary carcinoma and positive in terms of estrogen receptor (ER) expression. Cell kinetic parametersincluding labelling index, mitotic index and labelling index were assessed after exposure of the. FM3A cell line to0.001μg/ml of Tamoxifen and 0.25μg/ml of Femara for 4, 8, 16 and 32 h for all parameters. The results showedthat cell growth was inhibited by both agents. There was a significant decrease in labelling index and mitoticindex and significant increase in apoptotic index for all experimental groups. The differences between controland all experimental groups were statistically significant (p<0.001) for all applications.