The venous cord blood levels of free thyroxine (fT4), free triiodothyronine (fT3), reverse triiodothyronine (rT3), thyrotropin (TSH), thyroglobulin (TG) and thyroxine binding globulin (TBG) were studied in 56 mature and healthy newborns. Newborns with a gestational age less than 37 or more than 42 weeks, a delivery by forceps or cesarian section, a birth-weight less than 2500 g, a pH-value of the cord-artery blood less than 7.15, an Apgar-value after 1 minute less than 7 were excluded from the study. All mothers were non-smokers. The values of fT4 were 18.66 +/- 4.18 pmol/L, of fT3 were 1.59 +/- 0.75 pmol/L, of fT3 were 2152 +/- 666 pg/ml, of TSH were 7.83 +/- 4.49 mU/ml, of TG were 44.61 +/- 23.84 ng/ml, and of TBG were 25.61 +/- 5.42 micrograms/ml. A weak negative correlation was found between the TG-value and the pH-value of the cord-artery blood (r = -0.27, y = 191.55 - 22.82.x, p less than 0.05), and between the fT4 values and the gestational age (r = -0.34, y = 67.53-1.22.x, p = 0.01). The rT3-values were positively correlated to the gestational age (r = 0.29, y = -4571 + 167.x, p less than 0.03).