The progression of Lewis lung carcinoma has been examined in mice under the stress of different housing and experimental conditions. The maintenance of the animals in a low stress environment decreased the weight of spontaneous lung metastases in comparison with conventional housing. The handling of mice in the low stress environment for intraperitoneal saline administration increased metastasis formation, whereas the application of a psychological stressor (spatial disorientation) to these animals increased both primary tumor growth and metastasis formation. These results indicate that psychological and experimental stressors can modulate, presumably via neuroendocrine mechanisms, the host's antitumor responses which can control metastases and primary tumor independently from each other.