The lungs of BR6 breeding female mice which had developed spontaneous mammary tumours, were examined histologically for the presence of metastases. Metastatic nodules, or tumour cell emboli within blood vessels, were found in the lungs of 3 out of 42 (7 per cent) mice with pregnancy-dependent tumours, and in 33 out of 117 (28 per cent) mice with pregnancy-independent tumours. In general, primary tumours which were well-differentiated and organized were less likely to metastasize than poorly-differentiated ones, but there was an intermediate range within which the likelihood of metastasis could not be predicted. Analysis of the reproductive histories of the mice did not show any significant correlations between factors which might have influenced the growth and progression of the primary tumour and metastatic potential. However, the sustained presence of a high tumour load was associated with an increased probability of metastasis.