Single neonatal treatment with 25 μg vitamin D<sub>3</sub> significantly decreased the thymic glucocorticoid receptor density (B<sub>max</sub>) of 6-week-old male rats. In females, a similar treatment did not cause any changes. Single vitamin D<sub>3</sub> treatment (50 μg) during adolescence (i.e. 6-week-old animals) significantly increased the glucocorticoid receptor density in adult (10-week-old) males. No significant changes in receptor affinity (K<sub>d</sub>) could be observed. Considering that in earlier experiments similar neonatal treatments influenced bone mineral mass and sexual behavior, the hormonal imprinting effect of vitamin D<sub>3</sub> and its harmful effect on the development of other members of the steroid receptor superfamily, seems to be unquestionable.