The cortisol and corticosterone levels in the fetal and maternal plasma were measured during the last 10 days of gestation by a double isotope derivative assay. The cortisol levels in the fetal plasma increased from 0.89 μg/100 ml to 1.38 μg/100 ml between day 22 and 30 and decreased to 0.69 μg/100 ml in the newborn. Cortisol level in maternal plasma on day 22 (0.61 μg/100 ml) was higher than in the nonpregnant female (0.26 μg/100 ml) and a gradual increase was observed between day 22 and day 30 (1.83 μg/100 ml). Postpartum, the cortisol concentration decreased to nonpregnant levels. Plasma corticosterone levels fluctuated considerably both in the fetal and maternal plasma; the fetal plasma corticosterone decreased from 1.18 μg/100 ml on day 22 to 0.45 μg/100 ml on day 27 with an increase on day 28 (2.52 μg/100 ml) followed by a decrease on day 29 and 30. Blood corticosterone in the pregnant rabbit were similar to the values found in the nonpregnant female (2.42 μg/100 ml) up to day 26, but a decrease was observed on day 27 (1.40 μg/100 ml). The postpartum plasma corticosterone (1.06 μg/100 ml) was lower than that found in the nonpregnant female. There was a shift in the plasma corticosterone to cortisol ratio from 19:1 in the nonpregnant rabbit to 1.5:1 at 30 days of gestation. The percent unbound cortisol during gestation, determined by equilibrium dialysis, remained unchanged in the maternal serum (11–13%) hut increased from 7.8% to 16.2% in the fetal serum between days 24–30. After birth the corresponding values in the newborn and maternal serum were 36.3% and 22.7%, respectively. The total unbound cortisol present in the fetal and maternal plasma was calculated and a 4-fold increase in the total unbound cortisol was found between days 24 and 30. The rise in the concentration of unbound cortisol in the fetal blood near term correlates very well with several developing systems in the rabbit fetus which are known to be under the control of glucocorticoids. (Endocrinology93: 1342, 1973)