The soluble-insoluble collagen ratio was determined in aortas removed at autopsy during the period 1966-1970 (21-25 years post exposure); 261 specimens were examined from persons of both sexes, age 30-89 years at death. A portion of the test samples were from persons located less than 1500 meters from the hypocenter at the time of the bomb; the majority of such individuals are thought to have absorbed a significant amount of ionizing radiation. Over-all, the ratio in this group was not significantly different from a comparable cadre of individuals who were not in the cities of Hiroshima and Nagasaki at the time of the explosions. However, a possible discrepancy was noted between the two exposure groups involving both males and females who were less than 60 years of age at the time of death with lower average values among the exposed groups. An interpretation of the latter observation is discussed and related to the experimental situation where the life-shortening effects of radiation are most pronounced when the animals are exposed at a young age.