“…The adult nervous system is highly resistant to ionizing radiation at dosage levels which severely damage many other tissues (Prosser, 1947), Sublethal X irradiation of adult rats (Arnold, 1952;Furchtgott, 1951;Tait, Wall, Balrnuth, & Kaplan, 1952) and monkeys (Davis, McDowell, Deter, & Steele, 1956;Kaplan, Gentry, Melching, & Dclit, 1954;Moon, Harlow, & Bogumill, 1955) produces few demonstrable effects upon learning. However, irradiation of fetal rats has been found to produce marked decrements in the adult learning of these animals (Levinson, 1952;Tait et al, 1952), the extent of the deficit being a function of dosage level and age at irradiation. On the other hand, the performance of rats irradiated at three weeks of age seems to be as highly resistant to X irradiation as that of irradiated adult animals.…”