The effects of the size of an implanted tumor and of its time of growth on metastasis were examined in the same tumor grown under two different conditions. Tumors were implanted either in the untreated legs of mice or in legs that had been irradiated one day earlier. The radiation slowed the growth of tumors. When the tumors grew to predetermined diameters, the legs were excised and 20 days later the mice were examined for the presence of lung metastases. Equal changes in the time to excision of the tumors grown under the two conditions led to nearly equal changes in the proportions of animals with metastases. Tumor volume effects were not apparent in this consideration of changes in metastasis rates with time. This work reanalyses the data of that study using the variability in growth rates within a tumor growth condition to investigate the effect of tumor size on metastatic intensity. This intensity increased at least linearly with tumor volume for both irradiated and control animals. The finding of equal effect at equal growth times of the implanted tumors under the two growth conditions was confirmed but is due to: (1) metastatic intensity changing five times as rapidly with tumor size in the irradiated group as compared to the control group and (2) non-exponential retarded growth of the tumors in the control treatment group.