Oestrogen (ER) and progestin receptors (PR) were measured in cytosols from histologically normal mammary tissues (n = 30), and in benign (n = 59) and malignant mammary lesions (n = 49) from female dogs. Receptor levels greater than or equal to 5 fmol mg-1 protein were considered positive. The presence of histologically normal mammary epithelium within specimens of primary tumours was noticed as a factor that may cause false-positive receptor results. Receptor levels in non-malignant tissues, and the receptor status of primary cancers did not vary significantly with regard to the phase of oestrous cycle (anoestrus/metoestrus) or the influence of exogenous progestins. ER- or PR-positivity was more frequent and levels of both receptors were higher in 'normal' tissues and in benign lesions than in primary cancers (P less than 0.001). ER and PR levels were higher in benign lesions of dogs also developing malignant mammary tumours than in benign lesions of dogs that did not (P less than 0.02 and P less than 0.05, respectively). Regional and distant cancer metastases were frequently receptor-negative. In some dogs heterogeneity of receptor status was found between different sites of the same cancer. These findings indicate that in non-malignant mammary tissues of adult female dogs expression of the genes encoding ER and PR is common. In malignant tumours this property may become lost, in particular in advanced states of disease.