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Day-old guinea-pigs were given a non-paralysing dose of 10 Gy X-rays to the lumbar spinal cord. One year later there was no evidence of any residual radiation damage as the dose required to produce paralysis was the same for these animals as for others not previously irradiated. When given a single dose only, guinea-pigs irradiated when 1 day old became paralysed after lower doses and with shorter latencies than those irradiated at 1 year. When irradiated at 30 days of age, the dose inducing paralysis was the same as at 1 year of age, but the latency period was shorter and similar to guinea-pigs irradiated at 1 day old. Thus at 30 days of age, adult radiotolerance had been acquired but latencies were still as short as in neonates. Whatever the age at irradiation, changes in latency for paralysis were closely related to changes in histopathological lesions in the cord and both were related to dose. White matter necrosis always occurred after higher doses and after shorter latencies than diffuse vacuolar demyelination.
John F. Knowles (Sat,) studied this question.