112 rats ranging in age from 8 hours to 15 days received single doses of radiation directly to the head of 125, 300, 500, or 1000 r. The highest mortality was found in animals irradiated in the immediate newborn period. Nearly 70% of the rats irradiated under 5 days of age had neurologic findings, as compared to 15% of those irradiated after 5 days. The most common neurologic findings were tremors, incorrdination, leg dragging, inability to right themselves, and small head size. The adult body weight of the animals irradiated during the first 5 days of life was lower than that of animals irradiated after the 5th day. Cataracts developed in 7 rats, of which 6 were noted to have small heads. Cataract incidence was greatest in animals irradiated with 500 r on the 5th day. The low incidence of cataracts in the animals irradiated under 5 days of age may been due to the great number of early deaths in this group, before cataracts could develop. Small heads were observed in 23 animals.