AbstractRats aged 2–2½ years, whose cerebellums were focally irradiated with a variable number of daily does of 200 R X ray during infancy were tested on a number of qualitative behavioral observations (motor activity, ataxia, tremor, and emotionality). We noted that many of the behavioral abnormalities, in evidence in the preweaning observations which had then disappeared, reappeared again in old age. A treatment‐senescence interactions was suggested to account for the