The concentrations of T4, T3, rT3, and TSH were measured at term pregnancy in maternal and umbilical plasma and in amniotic fluid of 11 normal patients who received 8-16 mg dexamethasone 3-48 h before elective cesarean section and of 10 control patients who received no dexamethasone. The mean (+/- SE) concentrations of T4 (micrograms per dl) in maternal and umbilical plasma of dexamethasone-treated patients (12.5 +/- 0.9 and 13.0 +/- 0.9) were not significantly different (P less than 0.05) from those of the control patients (13.9 +/- 1.5 and 10.4 +/- 0.6, respectively). The mean (+/- SE) maternal plasma concentrations of T3 and rT3 (nanograms per dl) of dexamethasone-treated patients (204 +/- 6 and 82 +/- 11) were not significantly different (P less than 0.05) from those of the control patients (201 +/- 26 and 72 +/- 6, respectively). However, the mean (+/- SE) concentrations of T3 and rT3 (nanograms per dl) in umbilical plasma of dexamethasone-treated patients (106 +/- 13 and 360 +/- 35) were 3- and 2-fold and significantly higher (P less than 0.05) than those of the control group (39 +/- 6 and 195 +/- 19, respectively). No significant differences (P less than 0.05) were observed between the mean concentrations of TSH (microunits per ml) in maternal and umbilical plasma of dexamethasone-treated patients (2.5 +/- 0.5 and 3.0 +/- 1.0) and those of the control group (2.8 +/- 0.5 and 6.9 +/- 2.7, respectively). Under the conditions studied, no differences in the mean concentrations of amniotic fluid T4, T3, rT3, or TSH were observed between the two groups of patients (P less than 0.05). The increase of T3 and rT3 levels in umbilical plasma after dexamethasone administration indicates alteration in fetal thyroid economy.