Abstract
Neonatal male rats (aged 0–4 days) were exposed to 150 and 460 r x-rays and killed at the age of 25 days. A quantitative histological study of the testes showed that there is a decrease in the radiosensitivity of male germ-cells between 1 and 4 days. Peak radiosensitivity does not coincide with microscopically demonstrable signs of nuclear activity in the gonocytes.