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The change in the pattern of biotransformation of |14C|corticosterone in fetal mouse brain between gestational days 14 and 17 increased the proportion of unchanged hormone from 9-75% A sharp decrease in the in vitro incorporation of [14C]leucine, [3H|uridine, and [3H)thymidine into incubated brain coincided with this change and continued until day 19, when the incorporation of the 3 substrates had fallen to 9, 54, and 16% respectively, of that on day 14. Injection of dexamethasone reduced values on day 14 to those normally found on days 15-18. Enzymes which metabolize corticosteroids regulate their activity in specific tissues; these data suggest a hormonal influence on developing brain.Corticosteroids are known to induce metabolic processes in a number of fetal tissues; the brain can be included in these. These changes are mediated by enzymes in that tissue and do not necessarily involve the fetal pituitary-adrenal axis, nor the pattern of glucocorticoid receptors.