In studies designed to determine the role of hypoxia and age in the development of cardiac disease in animals, it was found that cardiac disease was induced by a single 24 hour exposure to a simulated altitude of 7620 meters and a single intravenous injection of Streptococcus sanguis, a microorganism known to colonize teeth surfaces. Within 6 days 54% of the exposed and 30% of the unexposed middle aged (10 months old) rats and about 15.0% of the young adults (2 months old) had bacterial endocarditis. Interstitial myocarditis developed in 63% of the injected exposed rats, and in none of the unexposed young adults. Interstitial nephritis was found in 46 and 66% of the injected unexposed young and middle aged rats, and in 70 and 86% of the injected, exposed young and middle aged rats respectively. About 95% of all injected rats survived 6 days. No evidence of hemo-concentration was found. The increase in cardiac disease induced by altitude was probably due to deleterious effects of hypoxia on the myocardium and cellular defenses, and to physiological and possibly immunological changes associated with aging.