Corticosterone in fetal mouse tissues after injection of mothers with 14C-corticosterone was determined by acetylation with 3H-acetic anhydride and cocrystallization to constant specific activity. The corticosterone content of whole fetal tissue varied between gestational days 13 and 17 from 641 to 300 ng/g. Specific activity of fetal hormone remained essentially constant; after a 15-min pulse this was as much as one-quarter that of maternal hormone. Placenta, head and liver showed distinctly different patterns which changed during this time, with a decrease in conversion of corticosterone to 11-dehydrocorticosterone. A sharp increase occurred in the activity of fetal liver 11beta-hydroxysteroid:NADP oxidoreductase activity. This mitochondrial enzyme, pH optimum 6, KM = 33 micrometer, reduced the metabolite, raising the relative amount of corticosterone in the fetus from 16 to 91%. 1 day after removal of maternal adrenals both maternal and fetal corticosterone were normal, indicating ability of fetal adrenals to function. Maternal corticosterone, however, crossed the placenta readily and it is considered likely that, normally, the maternal hormone predominates. Regardless of origin, corticosterone is maintained by enzymatic conversion in a distinct manner in different tissues.