Temporal and quantitative aspects of lymphogenous and hematogenous metastasis were examined using the rat MT-100-TC mammary carcinoma injected into the hind feet of syngeneic rats. Metastases first appeared in the draining popliteal nodes and then progressed in an invariable pattern to regional and then distal nodes: 'skipped' negative nodes within a chain of positive nodes were not observed. Metastatic progression in the lymphatic system occurred metachronously, with nodal metastases acting as 'generalizing' sites for 'downstream' nodes. Perturbation of lymph flow was apparent when nodes were involved with tumor, and resulted in retrograde seeding of contralateral nodes. Lung involvement was first observed by ectopic bioassay in 50 per cent of animals after 1 and 2 weeks of primary tumor growth; in contrast, all animals had popliteal involvement after 1 week. These results indicate that lymph nodes and lungs are not seeded synchronously, and the lungs are seeded after nodal metastases. Thus, a phase of metastasis has been identified, during which resection of the primary tumor and local nodes may well be curative in the 50 per cent of cases in which the disease is confined to these sites.