1. 1. Rats exposed to benzpyrene in utero at 19 days of pre-natal life showed a relative decrease in the number of thymic glucocorticoid receptors at 6 weeks of age. 2. 2. Primary exposure to benzpyrene at 6 weeks of age had a similar effect on females 4 weeks later, but did not change the glucocorticoid receptor number of males. 3. 3. In utero exposure accounted for an increase in the fetal cytochrome P450 level within 1 day, whereas exposure at 6 weeks of age did not change it within 4 weeks. 4. 4. It appears that exposure to benzpyrene gives rise to a faulty imprinting of the thymic glucocorticoid receptor in both fetal and growing age, to judge from a lasting change in the receptor number.
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