4-Hydroxytamoxifen is the active metabolite of tamoxifen and a selective estrogen receptor (ER) modulator that is widely used in the therapeutic and chemopreventive treatment of breast cancer. 4-Hydroxytamoxifen is a potent activator of ER and has been shown to have a higher affinity for estrogen receptors compared to tamoxifen. It has significant dose and time-dependent direct effects to alter the contractile function of isolated adult rat cardiac myocytes.