Evaluation of the treatment response to hormonal therapy in experimental mammary carcinoma models usually consists of recording the changes in tumour volume or in number of tumours. We performed quantitative histology on dimethylbenz[a]anthracene(DMBA) induced rat mammary carcinoma treated with the antioestrogen toremifene. The inhibition of growth of the treated regressing tumours was not only associated with inhibition of mitosis and reduction of the neoplastic epithelium, but toremifene seemed to reduce all main tissue components. The percentual area fractions of epithelium and stroma were equal in the untreated and the treated tumours. The estimated absolute volumes of epithelium, stroma and glandular luminae as well as the mitotic index were significantly reduced during toremifene treatment in those tumours which were responding by regression to the treatment. The findings indicate growth inhibiting effect on both epithelium and stroma or on interactions between epithelium and stroma. Adding quantitative histology to the gross response analysis in experimental animal models is feasible and may elucidate the mechanisms of action of a novel hormonal treatment.