High-dose tamoxifen citrate (80 mg/kg), or estradiol (100 micrograms/kg) was administered to intact or castrated Balb/C male and female mice with implanted murine renal cell adenocarcinoma. 100% tumor growth occurred, and tumor weights in tamoxifen treated (mean 3.69 g) and untreated (mean 3.11 g) groups were relatively the same. All estradiol-treated groups had slightly lower tumor weights (mean 2.31 g). This was, however, not statistically significant. Metastases occurred in all groups: control groups had 24-50%, tamoxifen-treated groups had 22-61%, and the estradiol-treated groups had 30-75%. In this murine renal adenocarcinoma model, tamoxifen did not appear to retard tumor growth or metastatic spread.