There is very little known about the subsequent behavior of breast cancer following adjuvant endocrine therapy or the effect of this therapy on the oestrogen and progesterone receptor content of recurrent disease. Mammary tumours were induced in Sprague-Dawley rats using N-nitrosomethylurea. In two experiments animals were treated with tamoxifen at different doses prior to the appearance of tumours. Tamoxifen prolonged the induction time and reduced the total number of tumours. Subsequent treatment of established tumours exposed to tamoxifen in the induction period demonstrated a reduced response to further tamoxifen therapy. There was no difference in the oestrogen receptor content of the tumours previously exposed to tamoxifen, but there was a significant reduction in the progesterone receptor content. The results suggest that: (a) patients treated with adjuvant tamoxifen therapy will show a delay in the appearance of recurrent disease with a reduced response to further tamoxifen therapy; (b) the model may provide an in vivo method to examine the relationships between oestrogen and progesterone receptors and response to endocrine therapy.