SummaryIn earlier studies both the incidence and spread of metastasis from spontaneous or subcutaneously transplanted tumors of mice were greater at high altitude than at sea level. The present experiments were designed to ascertain whether the growth of intravenously injected cells from a transplantable ascites tumor would differ at the two altitudes. Weanling C3H mice were distributed by litter and sex between the high-level and low-level locations. At the age of 10 weeks, mice of both groups received intravenous injections of 5 × 10⁴ of #678 ascites carcinoma cells. After injection, mice at each location were divided into four groups: 1) those permitted to live their lifespan and 2), 3), and 4) those killed at 15, 10, and 6 days, respectively, after injection. In all groups, there were a greater number of mice with “embolic takes” and a greater number of pulmonary tumor nodules per mouse at high altitude than at sea level. Mice survived longer at sea level than at high altitude.