Schematic presentation of the study model. 16 Sprague-Dawley rats with DMBA-induced endocrine sensitive breast cancer were randomized to treatment with oral tamoxifen or vehicle only for 14 days. The rats were weighed every third day for calculation of treatment dosage and the tumors were counted and measured. Blood was sampled at the end of the study for measurements of tamoxifen and metabolites, and tumor tissue was collected for gene expression measurements of SRCs and the growth factor receptors HER-1 to -4. Tumor tissue was also used for protein analyses of SRCs and HERs by Western blotting and for drug measurements using LC-MS/MS.