SIKOV ra 1014 Studies of factors that influence the response of the perinatal animal to radiation include studies to evaluate the role of genetic factors and of immunologic competence in the response of the neonatal animal. Other studies have indicated the relevance of usually ignored factors, such as season of the year. Relatively few investigators have been interested in the long-term effects of radiation on the intrauterine or neonatal animal. Many of the groups involved in this area of research are represented at this symposium, We have seen that the differences between the late effects of irradiation of the perinatal animal and of the adult provide clues to the unique nature of the response of the immature animal to radiation. Studies with radionuclides have also pointed out differences and have raised several, yet unanswered, questions about the mechanisms of effect. - Renewed interest in the effects of continuous or fractionated irradiation was evident. Although many of the studies reported were descriptive, this interest may signal coming advances since comparable studies in the adult led to some of our more useful working hypotheses on the mechanism of radiation effects. As an indication of the under- standing that may result, studies combining split-dose techniques for irradiation of the cerebrum with quantitative end points have suggested that partial recovery occurs in the ‘fetal brain. Many morphological studies are still being conducted, but there is a distinct trend toward elegance of approach. The increasing use of the electron microscope to investigate the ‘finer details of many of these morphologic changes has been evident. We also saw the improved understanding that has resulted from the use of fine beams of X rays or protons delivered to narrowly defined areas of the brain. Much of the current work is involved: with elucidating the mechanisms responsible for morphologic changes described in earlier studies, ” The increasingly quantitative approach contrasts markedly with earlier work, For example, growth is being measured as an increase in DNA rather ‘than in total weight. The effects of radiation on cell proliferation are being subjected to definitive scrutiny through the use of tritiated thymidine labeling. At least one group has carefully measured the size of important functional compartments, which has allowed them to propose quantitative explanations for age-dependent differences in radiosensitivity. The emphasis on quantitation also encompasses the physiological and biochemical changes that result from irradiation during the perinatal period. Reports included measurements of enzyme activities as well as protein, DNA, and RNA content. A few laboratories are carrying this a step further and are attempting to use such data to define the effects at the molecular level.