To understand the endocrine control of normal parturition in man and interactions of foetal and maternal hormones involved, it is desirable to estimate the hormone levels in the maternal and foetal blood around parturition time. Blood was obtained from the umbilical vessels of 22 male and 30 female neonates and the antecubital veins of their mothers during labour at term. The maternal and neonatal plasma levels of progesterone (P), testosterone (T) and estradiol-17 beta (E2) and maternal plasma levels of human chorionic gonadotrophin (hCG) were determined by radioimmunoassay (RIA). The effect of the foetal sex on the maternal hormones at term was also evaluated. Mean values, with the standard errors of means, of plasma, P, T and E2 in cord blood were 5.3 +/- 0.3, 8.4 +/- 1.2 and 5.0 +/- 0.3 ng/ml, respectively; in the maternal plasma the values were 18.2 +/- 1.2, 1.2 +/- 0.1 and 4.3 +/- 0.4 ng/ml, respectively. The mean T level was significantly higher in cord than in maternal blood, while P was significantly higher in the maternal than in cord blood. The plasma concentrations of P, T and E2 in the mothers delivering male babies were 16.9 +/- 2.2, 1.2 +/- 0.4 and 4.5 +/- 0.7 ng/ml, respectively; in those delivering female babies, the levels were 16.1 +/- 2.4, 1.1 +/- 0.1 and 4.3 +/- 0.4 ng/ml, respectively. Mean plasma levels of hCG in the women was 0.9 +/- 0.1 microgram/ml with no significant difference between the values in the mothers of male and female babies.(ABSTRACT TRUNCATED AT 250 WORDS)