The relation between the excretion of urinary steroids and the therapeutic effect of ovariectomy on tumors was investigated in female Sprague-Dawley rats bearing 7,12-dimethylbenz[a]anthracene-induced mammary cancer. Ovariectomy significantly reduced the excretion of urinary androgen, 11-deoxy-17-oxosteroid (11-DOOS), but hardly affected that of urinary corticosteroid, 17-deoxy-corticosteroid (17-DOCS). The responsiveness of tumors to ovariectomy was positively correlated with both pre-operative 11-DOOS and pre-operative ratio of 11-DOOS to 17-DOCS, and negatively correlated with pre-operative 17-DOCS. The extent of tumor regression after ovariectomy was associated with the decrease of urinary androgen. A similar correlation was observed between the decrease in the ratio 11-DOOS/17-DOCS and tumor suppression during cyclophosphamide treatment (medical ovariectomy). These results indicated that the tumor response to ovariectomy was related to a shift of the endogenous hormone balance from a relative dominance of androgen to relative dominance of corticosteroid. The significance of the hormonal findings is discussed in the light of the steroid metabolism in human breast cancer.